The Power of Movement in Plants

by Charles Darwin; Sir Francis Darwin

DIAGEOTROPISM.

Besides geotropism and apogeotropism, there is, according to Frank, an allied form of movement, namely, “transverse-geotropism,” or diageotropism, as we may call it for the sake of matching our other terms. Under the influence of gravitation certain parts are excited to place themselves more or less transversely to the line of its action.[^[8]] We made no observations on this subject, and will here only remark that the position of the secondary radicles of various plants, which extend horizontally or are a little inclined downwards, would probably be considered by Frank as due to transverse-geotropism. As it has been shown in Chap. I. that the secondary radicles of Cucurbita made serpentine tracks on a smoked glass-plate, they clearly circumnutated, and there can hardly be a doubt that this holds good with other secondary radicles. It seems therefore highly probable that they place themselves in their diageotropic position by means of modified circumnutation.

[8] Elfving has lately described (‘Arbeiten des Bot. Instituts in Würzburg,’ B. ii. 1880, p. 489) an excellent instance of such movements in the rhizomes of certain plants.

Finally, we may conclude that the three kinds of movement which have now been described and which are excited by gravitation, consist of modified circumnutation. Different parts or organs on the same plant, and the same part in different species, are thus excited to act in a widely different manner. We can see no reason why the attraction of gravity should directly modify the state of turgescence and subsequent growth of one part on the upper side and of another part on the lower side. We are therefore led to infer that both geotropic, apogeotropic, and diageotropic movements, the purpose of which we can generally understand, have been acquired for the advantage of the plant by the modification of the ever-present movement of circumnutation. This, however, implies that gravitation produces some effect on the young tissues sufficient to serve as a guide to the plant.

CHAPTER XI.
LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.

General considerations—Vicia faba, effects of amputating the tips of the radicles—Regeneration of the tips—Effects of a short exposure of the tips to geotropic action and their subsequent amputation—Effects of amputating the tips obliquely—Effects of cauterising the tips—Effects of grease on the tips—Pisum sativum, tips of radicles cauterised transversely, and on their upper and lower sides—Phaseolus, cauterisation and grease on the tips—Gossypium—Cucurbita, tips cauterised transversely, and on their upper and lower sides—Zea, tips cauterised—Concluding remarks and summary of chapter—Advantages of the sensibility to geotropism being localised in the tips of the radicles.

Ciesielski states[^[1]] that when the roots of Pisum, Lens and Vicia were extended horizontally with their tips cut off, they were not acted on by geotropism; but some days afterwards, when a new root-cap and vegetative point had been formed, they bent themselves perpendicularly downwards. He further states that if the tips are cut off, after the roots have been left extended horizontally for some little time, but before they have begun to bend downwards, they may be placed in any position, and yet will bend as if still acted on by geotropism; and this shows that some influence had been already transmitted to the bending part from the tip before it was amputated. Sachs repeated these experiments; he cut off a length of between .05 and 1 mm. (measured from the apex of the vegetative point) of the tips of the radicles of the bean (Vicia faba), and placed them horizontally or vertically in damp air, earth, and water, with the result that they became bowed in all sorts of directions.[^[2]] He therefore disbelieved in Ciesielski’s conclusions. But as we have seen with several plants that the tip of the radicle is sensitive to contact and to other irritants, and that it transmits some influence to the upper growing part causing it to bend, there seemed to us to be no a priori improbability in Ciesielski’s statements. We therefore determined to repeat his experiments, and to try others on several species by different methods.

[1] ‘Abwartskrümmung der Wurzel,’ Inaug. Dissert. Breslau, 1871, p. 29.

[2] ‘Arbeiten des Bot. Instituts in Würzburg,’ Heft. iii. 1873, p. 432.

Vicia faba.—Radicles of this plant were extended horizontally either over water or with their lower surfaces just touching it. Their tips had previously been cut off, in a direction as accurately transverse as could be done, to different lengths, measured from the apex of the root-cap, and which will be specified in each case. Light was always excluded. We had previously tried hundreds of unmutilated radicles under similar circumstances, and found that every one that was healthy became plainly geotropic in under 12 h. In the case of four radicles which had their tips cut off for a length of 1.5 mm., new root caps and new vegetative points were re-formed after an interval of 3 days 20 h.; and these when placed horizontally were acted on by geotropism. On some other occasions this regeneration of the tips and reacquired sensitiveness occurred within a somewhat shorter time. Therefore, radicles having their tips amputated should be observed in from 12 to 48 h. after the operation.

Four radicles were extended horizontally with their lower surfaces touching the water, and with their tips cut off for a length of only 0.5 mm.: after 23 h. three of them were still horizontal; after 47 h. one of the three became fairly geotropic; and after 70 h. the other two showed a trace of this action. The fourth radicle was vertically geotropic after 23 h.; but by an accident the root-cap alone and not the vegetative point was found to have been amputated; so that this case formed no real exception and might have been excluded.

Five radicles were extended horizontally like the last, and had their tips cut off for a length of 1 mm.; after 22–23 h., four of them were still horizontal, and one was slightly geotropic; after 48 h. the latter had become vertical; a second was also somewhat geotropic; two remained approximately horizontal; and the last or fifth had grown in a disordered manner, for it was inclined upwards at an angle of 65° above the horizon.

Fourteen radicles were extended horizontally at a little height over the water with their tips cut off for a length of 1.5 mm.; after 12 h. all were horizontal, whilst five control or standard specimens in the same jar were all bent greatly downwards. After 24 h. several of the amputated radicles remained horizontal, but some showed a trace of geotropism, and one was plainly geotropic, for it was inclined at 40° beneath the horizon.

Seven horizontally extended radicles from which the tips had been cut off for the unusual length of 2 mm. unfortunately were not looked at until 35 h. had elapsed; three were still horizontal, but to our surprise, four were more or less plainly geotropic.

The radicles in the foregoing cases were measured before their tips were amputated, and in the course of 24 h. they had all increased greatly in length; but the measurements are not worth giving. It is of more importance that Sachs found that the rate of growth of the different parts of radicles with amputated tips was the same as with unmutilated ones. Altogether twenty-nine radicles were operated on in the manner above described, and of these only a few showed any geotropic curvature within 24 h.; whereas radicles with unmutilated tips always became, as already stated, much bent down in less than half of this time. The part of the radicle which bends most lies at the distance of from 3 to 6 mm. from the tip, and as the bending part continues to grow after the operation, there does not seem any reason why it should not have been acted on by geotropism, unless its curvature depended on some influence transmitted from the tip. And we have clear evidence of such transmission in Ciesielski’s experiments, which we repeated and extended in the following manner.

Beans were embedded in friable peat with the hilum downwards, and after their radicles had grown perpendicularly down for a length of from ½ to 1 inch, sixteen were selected which were perfectly straight, and these were placed horizontally on the peat, being covered by a thin layer of it. They were thus left for an average period of 1 h. 37 m. The tips were then cut off transversely for a length of 1.5 mm., and immediately afterwards they were embedded vertically in the peat. In this position geotropism would not tend to induce any curvature, but if some influence had already been transmitted from the tip to the part which bends most, we might expect that this part would become curved in the direction in which geotropism had previously acted; for it should be noted that these radicles being now destitute of their sensitive tips, would not be prevented by geotropism from curving in any direction. The result was that of the sixteen vertically embedded radicles, four continued for several days to grow straight downwards, whilst twelve became more or less bowed laterally. In two of the twelve, a trace of curvature was perceptible in 3 h. 30 m., counting from the time when they had first been laid horizontally; and all twelve were plainly bowed in 6 h., and still more plainly in 9 h. In every one of them the curvature was directed towards the side which had been downwards whilst the radicles remained horizontal. The curvature extended for a length of from 5 to, in one instance, 8 mm., measured from the cut-off end. Of the twelve bowed radicles five became permanently bent into a right angle; the other seven were at first much less bent, and their curvature generally decreased after 24 h., but did not wholly disappear. This decrease of curvature would naturally follow, if an exposure of only 1 h. 37 m. to geotropism, served to modify the turgescence of the cells, but not their subsequent growth to the full extent. The five radicles which were rectangularly bent became fixed in this position, and they continued to grow out horizontally in the peat for a length of about 1 inch during from 4 to 6 days. By this time new tips had been formed; and it should be remarked that this regeneration occurred slower in the peat than in water, owing perhaps to the radicles being often looked at and thus disturbed. After the tips had been regenerated, geotropism was able to act on them, so that they now became bowed vertically downwards. An accurate drawing (Fig. 195) is given on the opposite page of one of these five radicles, reduced to half the natural size.

We next tried whether a shorter exposure to geotropism would suffice to produce an after-effect. Seven radicles were extended horizontally for an hour, instead of 1 h. 37 m. as in the former trial; and after their tips (1.5 mm. in length) had been amputated, they were placed vertically in damp peat. Of these, three were not in the least affected and continued for days to grow straight downwards. Four showed after 8 h. 30 m. a mere trace of curvature in the direction in which they had been acted on by geotropism; and in this respect they differed much from those which had been exposed for 1 h. 37 m., for many of the latter were plainly curved in 6 h. The curvature of one of these four radicles almost disappeared after 24 h. In the second, the curvature increased during two days and then decreased. the third radicle became permanently bent, so that its terminal part made an angle of about 45° with its original vertical direction. The fourth radicle became horizontal. These two, latter radicles continued during two more days to grow in the peat in the same directions, that is, at an angle of 45° beneath the horizon and horizontally. By the fourth morning new tips had been re-formed, and now geotropism was able to act on them again, and they became bent perpendicularly downwards, exactly as in the case of the five radicles described in the last paragraph and as is shown in (Fig. 195) here given.

Fig. 195. Vicia faba: radicle, rectangularly bent at A, after the amputation of the tip, due to the previous influence of geotropism. L, side of bean which lay on the peat, whilst geotropism acted on the radicle. A, point of chief curvature of the radicle, whilst standing vertically downwards. B, point of chief curvature after the regeneration of the tip, when geotropism again acted. C, regenerated tip.

Lastly, five other radicles were similarly treated, but were exposed to geotropism during only 45 m. After 8 h. 30 m. only one was doubtfully affected; after 24 h. two were just perceptibly curved towards the side which had been acted on by geotropism; after 48 h. the one first mentioned had a radius of curvature of 60 mm. That this curvature was due to the action of geotropism during the horizontal position of the radicle, was shown after 4 days, when a new tip had been re-formed, for it then grew perpendicularly downwards. We learn from this case that when the tips are amputated after an exposure to geotropism of only 45 m., though a slight influence is sometimes transmitted to the adjoining part of the radicle, yet this seldom suffices, and then only slowly, to induce even moderately well-pronounced curvature.

In the previously given experiments on 29 horizontally extended radicles with their tips amputated, only one grew irregularly in any marked manner, and this became bowed upwards at an angle of 65°. In Ciesielski’s experiments the radicles could not have grown very irregularly, for if they had done so, he could not have spoken confidently of the obliteration of all geotropic action. It is therefore remarkable that Sachs, who experimented on many radicles with their tips amputated, found extremely disordered growth to be the usual result. As horizontally extended radicles with amputated tips are sometimes acted on slightly by geotropism within a short time, and are often acted on plainly after one or two days, we thought that this influence might possibly prevent disordered growth, though it was not able to induce immediate curvature. Therefore 13 radicles, of which 6 had their tips amputated transversely for a length of 1.5 mm., and the other 7 for a length of only 0.5 mm., were suspended vertically in damp air, in which position they would not be affected by geotropism; but they exhibited no great irregularity of growth, whilst observed during 4 to 6 days. We next thought that if care were not taken in cutting off the tips transversely, one side of the stump might be irritated more than the other, either at first or subsequently during the regeneration of the tip, and that this might cause the radicle to bend to one side. It has also been shown in Chapter III. that if a thin slice be cut off one side of the tip of the radicle, this causes the radicle to bend from the sliced side. Accordingly, 30 radicles, with tips amputated for a length of 1.5 mm., were allowed to grow perpendicularly downwards into water. Twenty of them were amputated at an angle of 20° with a line transverse to their longitudinal axes; and such stumps appeared only moderately oblique. The remaining ten radicles were amputated at an angle of about 45°. Under these circumstances no less than 19 out of the 30 became much distorted in the course of 2 or 3 days. Eleven other radicles were similarly treated, excepting that only 1 mm. (including in this and all other cases the root-cap) was amputated; and of these only one grew much, and two others slightly distorted; so that this amount of oblique amputation was not sufficient. Out of the above 30 radicles, only one or two showed in the first 24 h. any distortion, but this became plain in the 19 cases on the second day, and still more conspicuous at the close of the third day, by which time new tips had been partially or completely regenerated. When therefore a new tip is reformed on an oblique stump, it probably is developed sooner on one side than on the other: and this in some manner excites the adjoining part to bend to one side. Hence it seems probable that Sachs unintentionally amputated the radicles on which he experimented, not strictly in a transverse direction.

This explanation of the occasional irregular growth of radicles with amputated tips, is supported by the results of cauterising their tips; for often a greater length on one side than on the other was unavoidably injured or killed. It should be remarked that in the following trials the tips were first dried with blotting-paper, and then slightly rubbed with a dry stick of nitrate of silver or lunar caustic. A few touches with the caustic suffice to kill the root-cap and some of the upper layers of cells of the vegetative point. Twenty-seven radicles, some young and very short, others of moderate length, were suspended vertically over water, after being thus cauterised. Of these some entered the water immediately, and others on the second day. The same number of uncauterised radicles of the same age were observed as controls. After an interval of three or four days the contrast in appearance between the cauterised and control specimens was wonderfully great. The controls had grown straight downwards, with the exception of the normal curvature, which we have called Sachs’ curvature. Of the 27 cauterised radicles, 15 had become extremely distorted; 6 of them grew upwards and formed hoops, so that their tips sometimes came into contact with the bean above; 5 grew out rectangularly to one side; only a few of the remaining 12 were quite straight, and some of these towards the close of our observations became hooked at their extreme lower ends. Radicles, extended horizontally instead of vertically, with their tips cauterised, also sometimes grew distorted, but not so commonly, as far as we could judge, as those suspended vertically; for this occurred with only 5 out of 19 radicles thus treated.

Instead of cutting off the tips, as in the first set of experiments, we next tried the effects of touching horizontally extended radicles with caustic in the manner just described. But some preliminary remarks must first be made. It may be objected that the caustic would injure the radicles and prevent them from bending; but ample evidence was given in Chapter III. that touching the tips of vertically suspended radicles with caustic on one side, does not stop their bending; on the contrary, it causes them to bend from the touched side. We also tried touching both the upper and the lower sides of the tips of some radicles of the bean, extended horizontally in damp friable earth. The tips of three were touched with caustic on their upper sides, and this would aid their geotropic bending; the tips of three were touched on their lower sides, which would tend to counteract the bending downwards; and three were left as controls. After 24 h. an independent observer was asked to pick out of the nine radicles, the two which were most and the two which were least bent; he selected as the latter, two of those which had been touched on their lower sides, and as the most bent, two of those which had been touched on the upper side. Hereafter analogous and more striking experiments with Pisum sativum and Cucurbita ovifera will be given. We may therefore safely conclude that the mere application of caustic to the tip does not prevent the radicles from bending.

In the following experiments, the tips of young horizontally extended radicles were just touched with a stick of dry caustic; and this was held transversely, so that the tip might be cauterised all round as symmetrically as possible. The radicles were then suspended in a closed vessel over water, kept rather cool, viz., 55°–59° F. This was done because we had found that the tips were more sensitive to contact under a low than under a high temperature; and we thought that the same rule might apply to geotropism. In one exceptional trial, nine radicles (which were rather too old, for they had grown to a length of from 3 to 5 cm.), were extended horizontally in damp friable earth, after their tips had been cauterised and were kept at too high a temperature, viz., of 68° F., or 20° C. The result in consequence was not so striking as in the subsequent cases for although when after 9 h. 40 m. six of them were examined, these did not exhibit any geotropic bending, yet after 24 h., when all nine were examined, only two remained horizontal, two exhibited a trace of geotropism, and five were slightly or moderately geotropic, yet not comparable in degree with the control specimens. Marks had been made on seven of these cauterised radicles at 10 mm. from the tips, which includes the whole growing portion; and after the 24 h. this part had a mean length of 37 mm., so that it had increased to more than 3½ times its original length; but it should be remembered that these beans had been exposed to a rather high temperature.

Nineteen young radicles with cauterised tips were extended at different times horizontally over water. In every trial an equal number of control specimens were observed. In the first trial, the tips of three radicles were lightly touched with the caustic for 6 or 7 seconds, which was a longer application than usual. After 23 h. 30 m. (temp. 55°–56° F.) these three radicles, A, B, C (Fig. 196), were still horizontal, whilst the three control specimens had become within 8 h. slightly geotropic, and strongly so (D, E, F) in 23 h. 30 m. A dot had been made on all six radicles at 10 mm. from their tips, when first placed horizontally. After the 23 h. 30 m. this terminal part, originally 10 mm. in length, had increased in the cauterised specimens to a mean length of 17.3 mm., and to 15.7 mm. in the control radicles, as shown in the figures by the unbroken transverse line; the dotted line being at 10 mm. from the apex. The control or uncauterised radicles, therefore, had actually grown less than the cauterised; but this no doubt was accidental, for radicles of different ages grow at different rates, and the growth of different individuals is likewise affected by unknown causes. The state of the tips of these three radicles, which had been cauterised for a rather longer time than usual, was as follows: the blackened apex, or the part which had been actually touched by the caustic, was succeeded by a yellowish zone, due probably to the absorption of some of the caustic; in A, both zones together were 1.1 mm. in length, and 1.4 mm. in diameter at the base of the yellowish zone; in B, the length of both was only 0.7 mm., and the diameter 0.7 mm.; in C, the length was 0.8 mm., and the diameter 1.2 mm.

Fig. 196. Vicia faba: state of radicles which had been extended horizontally for 23 h. 30 m.; A, B, C, tips touched with caustic; D, E, F, tips uncauterised. Lengths of radicles reduced to one-half scale, but by an accident the beans themselves not reduced in the same degree.

Three other radicles, the tips of which had been touched with caustic curing 2 or 3 seconds, remained (temp. 58°–59° F.) horizontal for 23 h.; the control radicles having, of course, become geotropic within this time. The terminal growing part, 10 mm. in length, of the cauterised radicles had increased in this interval to a mean length of 24.5 mm., and of the controls to a mean of 26 mm. A section of one of the cauterised tips showed that the blackened part was 0.5 mm. in length, of which 0.2 mm. extended into the vegetative point; and a faint discoloration could be detected even to 1.6 mm. from the apex of the root-cap.

In another lot of six radicles (temp. 55°–57° F.) the three control specimens were plainly geotropic in 8½ h.; and after 24 h. the mean length of their terminal part had increased from 10 mm. to 21 mm. When the caustic was applied to the three cauterised specimens, it was held quite motionless during 5 seconds, and the result was that the black marks were extremely minute. Therefore, caustic was again applied, after 8½ h., during which time no geotropic action had occurred. When the specimens were re-examined after an additional interval of 15½ h., one was horizontal and the other two showed, to our surprise, a trace of geotropism which in one of them soon afterwards became strongly marked; but in this latter specimen the discoloured tip was only 2/3 mm. in length. The growing part of these three radicles increased in 24 h. from 10 mm. to an average of 16.5 mm.

It would be superfluous to describe in detail the behaviour of the 10 remaining cauterised radicles. The corresponding control specimens all became geotropic in 8 h. Of the cauterised, 6 were first looked at after 8 h., and one alone showed a trace of geotropism; 4 were first looked at after 14 h., and one alone of these was slightly geotropic. After 23–24h., 5 of the 10 were still horizontal, 4 slightly, and 1 decidedly, geotropic. After 48 h. some of them became strongly geotropic. The cauterised radicles increased greatly in length, but the measurements are not worth giving.

As five of the last-mentioned cauterised radicles had become in 24 h. somewhat geotropic, these (together with three which were still horizontal) had their positions reversed, so that their tips were now a little upturned, and they were again touched with caustic. After 24 h. they showed no trace of geotropism; whereas the eight corresponding control specimens, which had likewise been reversed, in which position the tips of several pointed to the zenith, all became geotropic; some having passed in the 24 h. through an angle of 180°, others through about 135°, and others through only 90°. The eight radicles, which had been twice cauterised, were observed for an additional day (i.e. for 48 h. after being reversed), and they still showed no signs of geotropism. Nevertheless, they continued to grow rapidly; four were measured 24 h. after being reversed, and they had in this time increased in length between 8 and 11 mm.; the other four were measured 48 h. after being reversed, and these had increased by 20, 18, 23, and 28 mm.

In coming to a conclusion with respect to the effects of cauterising the tips of these radicles, we should bear in mind, firstly, that horizontally extended control radicles were always acted on by geotropism, and became somewhat bowed downwards in 8 or 9 h.; secondly, that the chief seat of the curvature lies at a distance of from 3 to 6 mm. from the tip; thirdly, that the tip was discoloured by the caustic rarely for more than 1 mm. in length; fourthly, that the greater number of the cauterised radicles, although subjected to the full influence of geotropism during the whole time, remained horizontal for 24 h., and some for twice as long; and that those which did become bowed were so only in a slight degree; fifthly, that the cauterised radicles continued to grow almost, and sometimes quite, as well as the uninjured ones along the part which bends most. And lastly, that a touch on the tip with caustic, if on one side, far from preventing curvature, actually induces it. Bearing all these facts in mind, we must infer that under normal conditions the geotropic curvature of the root is due to an influence transmitted from the apex to the adjoining part where the bending takes place; and that when the tip of the root is cauterised it is unable to originate the stimulus necessary to produce geotropic curvature.

As we had observed that grease was highly injurious to some plants, we determined to try its effects on radicles. When the cotyledons of Phalaris and Avena were covered with grease along one side, the growth of this side was quite stopped or greatly checked, and as the opposite side continued to grow, the cotyledons thus treated became bowed towards the greased side. This same matter quickly killed the delicate hypocotyls and young leaves of certain plants. The grease which we employed was made by mixing lamp-black and olive oil to such a consistence that it could be laid on in a thick layer. The tips of five radicles of the bean were coated with it for a length of 3 mm., and to our surprise this part increased in length in 23 h. to 7.1 mm.; the thick layer of grease being curiously drawn out. It thus could not have checked much, if at all, the growth of the terminal part of the radicle. With respect to geotropism, the tips of seven horizontally extended radicles were coated for a length of 2 mm., and after 24 h. no clear difference could be perceived between their downward curvature and that of an equal number of control specimens. The tips of 33 other radicles were coated on different occasions for a length of 3 mm.; and they were compared with the controls after 8 h., 24 h., and 48 h. On one occasion, after 24 h., there was very little difference in curvature between the greased and control specimens; but generally the difference was unmistakable, those with greased tips being considerably less curved downwards. The whole growing part (the greased tips included) of six of these radicles was measured and was found to have increased in 23 h. from 10 mm. to a mean length of 17.7 mm.; whilst the corresponding part of the controls had increased to 20.8 mm. It appears therefore, that although the tip itself, when greased, continues to grow, yet the growth of the whole radicle is somewhat checked, and that the geotropic curvature of the upper part, which was free from grease, was in most cases considerably lessened.

Pisum sativum.—Five radicles, extended horizontally over water, had their tips lightly touched two or three times with dry caustic. These tips were measured in two cases, and found to be blackened for a length of only half a millimeter. Five other radicles were left as controls. The part which is most bowed through geotropism lies at a distance of several millimeters from the apex. After 24 h., and again after 32 h. from the commencement, four of the cauterised radicles were still horizontal, but one was plainly geotropic, being inclined at 45° beneath the horizon. The five controls were somewhat geotropic after 7 h. 20 m., and after 24 h. were all strongly geotropic; being inclined at the following angles beneath the horizon, viz., 59°, 60°, 65°, 57°, and 43°. The length of the radicles was not measured in either set, but it was manifest that the cauterised radicles had grown greatly.

The following case proves that the action of the caustic by itself does not prevent the curvature of the radicle. Ten radicles were extended horizontally on and beneath a layer of damp friable peat-earth; and before being extended their tips were touched with dry caustic on the upper side. Ten other radicles similarly placed were touched on the lower side; and this would tend to make them bend from the cauterised side; and therefore, as now placed, upwards, or in opposition to geotropism. Lastly, ten uncauterised radicles were extended horizontally as controls. After 24 h. all the latter were geotropic; and the ten with their tips cauterised on the upper side were equally geotropic; and we believe that they became curved downwards before the controls. The ten which had been cauterised on the lower side presented a widely different appearance: No. 1, however, was perpendicularly geotropic, but this was no real exception, for on examination under the microscope, there was no vestige of a coloured mark on the tip, and it was evident that by a mistake it had not been touched with the caustic. No. 2 was plainly geotropic, being inclined at about 45° beneath the horizon; No. 3 was slightly, and No. 4 only just perceptibly geotropic; Nos. 5 and 6 were strictly horizontal; and the four remaining ones were bowed upwards, in opposition to geotropism. In these four cases the radius of the upward curvatures (according to Sachs’ cyclometer) was 5 mm., 10 mm., 30 mm., and 70 mm. This curvature was distinct long before the 24 h. had elapsed, namely, after 8 h. 45 m. from the time when the lower sides of the tips were touched with the caustic.

Phaseolus multiflorus.—Eight radicles, serving as controls, were extended horizontally, some in damp friable peat and some in damp air. They all became (temp 20°–21° C.) plainly geotropic in 8 h. 30 m., for they then stood at an average angle of 63° beneath the horizon. A rather greater length of the radicle is bowed downwards by geotropism than in the case of Vicia faba, that is to say, rather more than 6 mm. as measured from the apex of the root-cap. Nine other radicles were similarly extended, three in damp peat and six in damp air, and dry caustic was held transversely to their tips during 4 or 5 seconds. Three of their tips were afterwards examined: in (1) a length of 0.68 mm. was discoloured, of which the basal 0.136 mm. was yellow, the apical part being black; in (2) the discoloration was 0.65 mm. in length, of which the basal 0.04 mm. was yellow; in (3) the discoloration was 0.6 mm. in length, of which the basal 0.13 mm. was yellow. Therefore less than 1 mm. was affected by the caustic, but this sufficed almost wholly to prevent geotropic action; for after 24 h. one alone of the nine cauterised radicles became slightly geotropic, being now inclined at 10° beneath the horizon; the eight others remained horizontal, though one was curved a little laterally.

The terminal part (10 mm. in length) of the six cauterised radicles in the damp air, had more than doubled in length in the 24 h., for this part was now on an average 20.7 mm. long. The increase in length within the same time was greater in the control specimens, for the terminal part had grown on an average from 10 mm. to 26.6 mm. But as the cauterised radicles had more than doubled their length in the 24 h., it is manifest that they had not been seriously injured by the caustic. We may here add that when experimenting on the effects of touching one side of the tip with caustic, too much was applied at first, and the whole tip (but we believe not more than 1 mm. in length) of six horizontally extended radicles was killed, and these continued for two or three days to grow out horizontally.

Many trials were made, by coating the tips of horizontally extended radicles with the before described thick grease. The geotropic curvature of 12 radicles, which were thus coated for a length of 2 mm., was delayed during the first 8 or 9 h., but after 24 h. was nearly as great as that of the control specimens. The tips of nine radicles were coated for a length of 3 mm., and after 7 h. 10 m. these stood at an average angle of 30° beneath the horizon, whilst the controls stood at an average of 54°. After 24 h. the two lots differed but little in their degree of curvature. In some other trials, however, there was a fairly well-marked difference after 24 h. between those with greased tips and the controls. The terminal part of eight control specimens increased in 24 h. from 10 mm. to a mean length of 24.3 mm., whilst the mean increase of those with greased tips was 20.7 mm. The grease, therefore, slightly checked the growth of the terminal part, but this part was not much injured; for several radicles which had been greased for a length of 2 mm. continued to grow during seven days, and were then only a little shorter than the controls. The appearance presented by these radicles after the seven days was very curious, for the black grease had been drawn out into the finest longitudinal striae, with dots and reticulations, which covered their surfaces for a length of from 26 to 44 mm., or of 1 to 1.7 inch. We may therefore conclude that grease on the tips of the radicles of this Phaseolus somewhat delays and lessens the geotropic curvature of the part which ought to bend most.

Gossypium herbaceum.—The radicles of this plant bend, through the action of geotropism, for a length of about 6 mm. Five radicles, placed horizontally in damp air, had their tips touched with caustic, and the discoloration extended for a length of from 2/3 to 1 mm. They showed, after 7 h. 45 m. and again after 23 h., not a trace of geotropism; yet the terminal portion, 9 mm. in length, had increased on an average to 15.9 mm. Six control radicles, after 7 h. 45 m., were all plainly geotropic, two of them being vertically dependent, and after 23 h. all were vertical, or nearly so.

Cucurbita ovifera.—A large number of trials proved almost useless, from the three following causes: Firstly, the tips of radicles which have grown somewhat old are only feebly geotropic if kept in damp air; nor did we succeed well in our experiments, until the germinating seeds were placed in peat and kept at a rather high temperature. Secondly, the hypocotyls of the seeds which were pinned to the lids of the jars gradually became arched; and, as the cotyledons were fixed, the movement of the hypocotyl affected the position of the radicle, and caused confusion. Thirdly, the point of the radicle is so fine that it is difficult not to cauterise it either too much or too little. But we managed generally to overcome this latter difficulty, as the following experiments show, which are given to prove that a touch with caustic on one side of the tip does not prevent the upper part of the radicle from bending. Ten radicles were laid horizontally beneath and on damp friable peat, and their tips were touched with caustic on the upper side. After 8 h. all were plainly geotropic, three of them rectangularly; after 19 h. all were strongly geotropic, most of them pointing perpendicularly downwards. Ten other radicles, similarly placed, had their tips touched with caustic on the lower side; after 8 h. three were slightly geotropic, but not nearly so much so as the least geotropic of the foregoing specimens; four remained horizontal; and three were curved upwards in opposition to geotropism. After 19 h. the three which were slightly geotropic had become strongly so. Of the four horizontal radicles, one alone showed a trace of geotropism; of the three up-curved radicles, one retained this curvature, and the other two had become horizontal.

The radicles of this plant, as already remarked, do not succeed well in damp air, but the result of one trial may be briefly given. Nine young radicles between .3 and .5 inch in length, with their tips cauterised and blackened for a length never exceeding ½ mm., together with eight control specimens, were extended horizontally in damp air. After an interval of only 4 h. 10 m. all the controls were slightly geotropic, whilst not one of the cauterised specimens exhibited a trace of this action. After 8 h. 35 m., there was the same difference between the two sets, but rather more strongly marked. By this time both sets had increased greatly in length. The controls, however, never became much more curved downwards; and after 24 h. there was no great difference between the two sets in their degree of curvature.

Eight young radicles of nearly equal length (average .36 inch) were placed beneath and on peat-earth, and were exposed to a temp. of 75°–76° F. Their tips had been touched transversely with caustic, and five of them were blackened for a length of about 0.5 mm., whilst the other three were only just visibly discoloured. In the same box there were 15 control radicles, mostly about .36 inch in length, but some rather longer and older, and therefore less sensitive. After 5 h., the 15 control radicles were all more or less geotropic: after 9 h., eight of them were bent down beneath the horizon at various angles between 45° and 90°, the remaining seven being only slightly geotropic: after 25 h. all were rectangularly geotropic. The state of the eight cauterised radicles after the same intervals of time was as follows: after 5 h. one alone was slightly geotropic, and this was one with the tip only a very little discoloured: after 9 h. the one just mentioned was rectangularly geotropic, and two others were slightly so, and these were the three which had been scarcely affected by the caustic; the other five were still strictly horizontal. After 24 h. 40 m. the three with only slightly discoloured tips were bent down rectangularly; the other five were not in the least affected, but several of them had grown rather tortuously, though still in a horizontal plane. The eight cauterised radicles which had at first a mean length of .36 inch, after 9 h. had increased to a mean length of .79 inch; and after 24 h. 40 m. to the extraordinary mean length of 2 inches. There was no plain difference in length between the five well cauterised radicles which remained horizontal, and the three with slightly cauterised tips which had become abruptly bent down. A few of the control radicles were measured after 25 h., and they were on an average only a little longer than the cauterised, viz., 2.19 inches. We thus see that killing the extreme tip of the radicle of this plant for a length of about 0.5 mm., though it stops the geotropic bending of the upper part, hardly interferes with the growth of the whole radicle.

In the same box with the 15 control specimens, the rapid geotropic bending and growth of which have just been described, there were six radicles, about .6 inch in length, extended horizontally, from which the tips had been cut off in a transverse direction for a length of barely 1 mm. These radicles were examined after 9 h. and again after 24 h. 40 m., and they all remained horizontal. They had not become nearly so tortuous as those above described which had been cauterised. The radicles with their tips cut off had grown in the 24 h. 40 m. as much, judging by the eye, as the cauterised specimens.

Zea mays.—The tips of several radicles, extended horizontally in damp air, were dried with blotting-paper and then touched in the first trial during 2 or 3 seconds with dry caustic; but this was too long a contact, for the tips were blackened for a length of rather above 1 mm. They showed no signs of geotropism after an interval of 9 h., and were then thrown away. In a second trial the tips of three radicles were touched for a shorter time, and were blackened for a length of from 0.5 to 0.75 mm.: they all remained horizontal for 4 h., but after 8 h. 30 m. one of them, in which the blackened tip was only 0.5 mm. in length, was inclined at 21° beneath the horizon. Six control radicles all became slightly geotropic in 4 h., and strongly so after 8 h. 30 m., with the chief seat of curvature generally between 6 or 7 mm. from the apex. In the cauterised specimens, the terminal growing part, 10 mm. in length, increased during the 8 h. 30 m. to a mean length of 13 mm.; and in the controls to 14.3 mm.

In a third trial the tips of five radicles (exposed to a temp. of 70°–71°) were touched with the caustic only once and very slightly; they were afterwards examined under the microscope, and the part which was in any way discoloured was on an average .76 mm. in length. After 4 h. 10 m. none were bent; after 5 h. 45 m., and again after 23 h. 30 m., they still remained horizontal, excepting one which was now inclined 20° beneath the horizon. The terminal part, 10 mm. in length, had increased greatly in length during the 23 h. 30 m., viz., to an average of 26 mm. Four control radicles became slightly geotropic after the 4 h. 10 m., and plainly so after the 5 h. 45 m. Their mean length after the 23 h. 30 m. had increased from 10 mm. to 31 mm. Therefore a slight cauterisation of the tip checks slightly the growth of the whole radicle, and manifestly stops the bending of that part which ought to bend most under the influence of geotropism, and which still continues to increase greatly in length.]

Concluding Remarks.—Abundant evidence has now been given, showing that with various plants the tip of the radicle is alone sensitive to geotropism; and that when thus excited, it causes the adjoining parts to bend. The exact length of the sensitive part seems to be somewhat variable, depending in part on the age of the radicle; but the destruction of a length of from less than 1 to 1.5 mm. (about 1/20th of an inch), in the several species observed, generally sufficed to prevent any part of the radicle from bending within 24 h., or even for a longer period. The fact of the tip alone being sensitive is so remarkable a fact, that we will here give a brief summary of the foregoing experiments. The tips were cut off 29 horizontally extended radicles of Vicia faba, and with a few exceptions they did not become geotropic in 22 or 23 h., whilst unmutilated radicles were always bowed downwards in 8 or 9 h. It should be borne in mind that the mere act of cutting off the tip of a horizontally extended radicle does not prevent the adjoining parts from bending, if the tip has been previously exposed for an hour or two to the influence of geotropism. The tip after amputation is sometimes completely regenerated in three days; and it is possible that it may be able to transmit an impulse to the adjoining parts before its complete regeneration. The tips of six radicles of Cucurbita ovifera were amputated like those of Vicia faba; and these radicles showed no signs of geotropism in 24 h.; whereas the control specimens were slightly affected in 5 h., and strongly in 9 h.

With plants belonging to six genera, the tips of the radicles were touched transversely with dry caustic; and the injury thus caused rarely extended for a greater length than 1 mm., and sometimes to a less distance, as judged by even the faintest discoloration. We thought that this would be a better method of destroying the vegetative point than cutting it off; for we knew, from many previous experiments and from some given in the present chapter, that a touch with caustic on one side of the apex, far from preventing the adjoining part from bending, caused it to bend. In all the following cases, radicles with uncauterised tips were observed at the same time and under similar circumstances, and they became, in almost every instance, plainly bowed downwards in one-half or one-third of the time during which the cauterised specimens were observed. With Vicia faba 19 radicles were cauterised; 12 remained horizontal during 23–24 h.; 6 became slightly and 1 strongly geotropic. Eight of these radicles were afterwards reversed, and again touched with caustic, and none of them became geotropic in 24 h., whilst the reversed control specimens became strongly bowed downwards within this time. With Pisum sativum, five radicles had their tips touched with caustic, and after 32 h. four were still horizontal. The control specimens were slightly geotropic in 7 h. 20 m., and strongly so in 24 h. The tips of 9 other radicles of this plant were touched only on the lower side, and 6 of them remained horizontal for 24 h., or were upturned in opposition to geotropism; 2 were slightly, and 1 plainly geotropic. With Phaseolus multiflorus, 15 radicles were cauterised, and 8 remained horizontal for 24 h.; whereas all the controls were plainly geotropic in 8 h. 30 m. Of 5 cauterised radicles of Gossypium herbaceum, 4 remained horizontal for 23 h. and 1 became slightly geotropic; 6 control radicles were distinctly geotropic in 7 h. 45 m. Five radicles of Cucurbita ovifera remained horizontal in peat-earth during 25 h., and 9 remained so in damp air during 8½ h.; whilst the controls became slightly geotropic in 4 h. 10 m. The tips of 10 radicals of this plant were touched on their lower sides, and 6 of them remained horizontal or were upturned after 19 h., 1 being slightly and 3 strongly geotropic.

Lastly, the tips of several radicles of Vicia faba and Phaseolus multiflorus were thickly coated with grease for a length of 3 mm. This matter, which is highly injurious to most plants, did not kill or stop the growth of the tips, and only slightly lessened the rate of growth of the whole radicle; but it generally delayed a little the geotropic bending of the upper part.

The several foregoing cases would tell us nothing, if the tip itself was the part which became most bent; but we know that it is a part distant from the tip by some millimeters which grows quickest, and which, under the influence of geotropism, bends most. We have no reason to suppose that this part is injured by the death or injury of the tip; and it is certain that after the tip has been destroyed this part goes on growing at such a rate, that its length was often doubled in a day. We have also seen that the destruction of the tip does not prevent the adjoining part from bending, if this part has already received some influence from the tip. As with horizontally extended radicles, of which the tip has been cut off or destroyed, the part which ought to bend most remains motionless for many hours or days, although exposed at right angles to the full influence of geotropism, we must conclude that the tip alone is sensitive to this power, and transmits some influence or stimulus to the adjoining parts, causing them to bend. We have direct evidence of such transmission; for when a radicle was left extended horizontally for an hour or an hour and a half, by which time the supposed influence will have travelled a little distance from the tip, and the tip was then cut off, the radicle afterwards became bent, although placed perpendicularly. The terminal portions of several radicles thus treated continued for some time to grow in the direction of their newly-acquired curvature; for as they were destitute of tips, they were no longer acted on by geotropism. But after three or four days when new vegetative points were formed, the radicles were again acted on by geotropism, and now they curved themselves perpendicularly downwards. To see anything of the above kind in the animal kingdom, we should have to suppose than an animal whilst lying down determined to rise up in some particular direction; and that after its head had been cut off, an impulse continued to travel very slowly along the nerves to the proper muscles; so that after several hours the headless animal rose up in the predetermined direction.

As the tip of the radicle has been found to be the part which is sensitive to geotropism in the members of such distinct families as the Leguminosae, Malvaceae, Cucurbitaceæ and Gramineæ, we may infer that this character is common to the roots of most seedling plants. Whilst a root is penetrating the ground, the tip must travel first; and we can see the advantage of its being sensitive to geotropism, as it has to determine the course of the whole root. Whenever the tip is deflected by any subterranean obstacle, it will also be an advantage that a considerable length of the root should be able to bend, more especially as the tip itself grows slowly and bends but little, so that the proper downward course may be soon recovered. But it appears at first sight immaterial whether this were effected by the whole growing part being sensitive to geotropism, or by an influence transmitted exclusively from the tip. We should, however, remember that it is the tip which is sensitive to the contact of hard objects, causing the radicle to bend away from them, thus guiding it along the lines of least resistance in the soil. It is again the tip which is alone sensitive, at least in some cases, to moisture, causing the radicle to bend towards its source. These two kinds of sensitiveness conquer for a time the sensitiveness to geotropism, which, however, ultimately prevails. Therefore, the three kinds of sensitiveness must often come into antagonism; first one prevailing, and then another; and it would be an advantage, perhaps a necessity, for the interweighing and reconciling of these three kinds of sensitiveness, that they should be all localised in the same group of cells which have to transmit the command to the adjoining parts of the radicle, causing it to bend to or from the source of irritation.

Finally, the fact of the tip alone being sensitive to the attraction of gravity has an important bearing on the theory of geotropism. Authors seem generally to look at the bending of a radicle towards the centre of the earth, as the direct result of gravitation, which is believed to modify the growth of the upper or lower surfaces, in such a manner as to induce curvature in the proper direction. But we now know that it is the tip alone which is acted on, and that this part transmits some influence to the adjoining parts, causing them to curve downwards. Gravity does not appear to act in a more direct manner on a radicle, than it does on any lowly organised animal, which moves away when it feels some weight or pressure.

CHAPTER XII.
CONCLUDING REMARKS.

Nature of the circumnutating movement—History of a germinating seed—The radicle first protrudes and circumnutates—Its tip highly sensitive—Emergence of the hypocotyl or of the epicotyl from the ground under the form of an arch–Its circumnutation and that of the cotyledons—The seedling throws up a leaf-bearing stem—The circumnutation of all the parts or organs—Modified circumnutation—Epinasty and hyponasty—Movements of climbing plants—Nyctitropic movements—Movements excited by light and gravitation—Localised sensitiveness—Resemblance between the movements of plants and animals—The tip of the radicle acts like a brain.

It may be useful to the reader if we briefly sum up the chief conclusions, which, as far as we can judge, have been fairly well established by the observations given in this volume. All the parts or organs in every plant whilst they continue to grow, and some parts which are provided with pulvini after they have ceased to grow, are continually circumnutating. This movement commences even before the young seedling has broken through the ground. The nature of the movement and its causes, as far as ascertained, have been briefly described in the Introduction. Why every part of a plant whilst it is growing, and in some cases after growth has ceased, should have its cells rendered more turgescent and its cell-walls more extensile first on one side and then on another, thus inducing circumnutation is not known. It would appear as if the changes in the cells required periods of rest.

In some cases, as with the hypocotyls of Brassica, the leaves of Dionaea and the joints of the Gramineæ, the circumnutating movement when viewed under the microscope is seen to consist of innumerable small oscillations. The part under observation suddenly jerks forwards for a length of .002 to .001 of an inch, and then slowly retreats for a part of this distance; after a few seconds it again jerks forwards, but with many intermissions. The retreating movement apparently is due to the elasticity of the resisting tissues. How far this oscillatory movement is general we do not know, as not many circumnutating plants were observed by us under the microscope; but no such movement could be detected in the case of Drosera with a 2-inch object-glass which we used. The phenomenon is a remarkable one. The whole hypocotyl of a cabbage or the whole leaf of a Dionaea could not jerk forwards unless a very large number of cells on one side were simultaneously affected. Are we to suppose that these cells steadily become more and more turgescent on one side, until the part suddenly yields and bends, inducing what may be called a microscopically minute earthquake in the plant; or do the cells on one side suddenly become turgescent in an intermittent manner; each forward movement thus caused being opposed by the elasticity of the tissues?

Circumnutation is of paramount importance in the life of every plant; for it is through its modification that many highly beneficial or necessary movements have been acquired. When light strikes one side of a plant, or light changes into darkness, or when gravitation acts on a displaced part, the plant is enabled in some unknown manner to increase the always varying turgescence of the cells on one side; so that the ordinary circumnutating movement is modified, and the part bends either to or from the exciting cause; or it may occupy a new position, as in the so-called sleep of leaves. The influence which modifies circumnutation may be transmitted from one part to another. Innate or constitutional changes, independently of any external agency, often modify the circumnutating movements at particular periods of the life of the plant. As circumnutation is universally present, we can understand how it is that movements of the same kind have been developed in the most distinct members of the vegetable series. But it must not be supposed that all the movements of plants arise from modified circumnutation; for, as we shall presently see, there is reason to believe that this is not the case.

Having made these few preliminary remarks, we will in imagination take a germinating seed, and consider the part which the various movements play in the life-history of the plant. The first change is the protrusion of the radicle, which begins at once to circumnutate. This movement is immediately modified by the attraction of gravity and rendered geotropic. The radicle, therefore, supposing the seed to be lying on the surface, quickly bends downwards, following a more or less spiral course, as was seen on the smoked glass-plates. Sensitiveness to gravitation resides in the tip; and it is the tip which transmits some influence to the adjoining parts, causing them to bend. As soon as the tip, protected by the root-cap, reaches the ground, it penetrates the surface, if this be soft or friable; and the act of penetration is apparently aided by the rocking or circumnutating movement of the whole end of the radicle. If the surface is compact, and cannot easily be penetrated, then the seed itself, unless it be a heavy one, is displaced or lifted up by the continued growth and elongation of the radicle. But in a state of nature seeds often get covered with earth or other matter, or fall into crevices, etc., and thus a point of resistance is afforded, and the tip can more easily penetrate the ground. But even with seeds lying loose on the surface there is another aid: a multitude of excessively fine hairs are emitted from the upper part of the radicle, and these attach themselves firmly to stones or other objects lying on the surface, and can do so even to glass; and thus the upper part is held down whilst the tip presses against and penetrates the ground. The attachment of the root-hairs is effected by the liquefaction of the outer surface of the cellulose walls, and by the subsequent setting hard of the liquefied matter. This curious process probably takes place, not for the sake of the attachment of the radicles to superficial objects, but in order that the hairs may be brought into the closest contact with the particles in the soil, by which means they can absorb the layer of water surrounding them, together with any dissolved matter.

After the tip has penetrated the ground to a little depth, the increasing thickness of the radicle, together with the root-hairs, hold it securely in its place; and now the force exerted by the longitudinal growth of the radicle drives the tip deeper into the ground. This force, combined with that due to transverse growth, gives to the radicle the power of a wedge. Even a growing root of moderate size, such as that of a seedling bean, can displace a weight of some pounds. It is not probable that the tip when buried in compact earth can actually circumnutate and thus aid its downward passage, but the circumnutating movement will facilitate the tip entering any lateral or oblique fissure in the earth, or a burrow made by an earth-worm or larva; and it is certain that roots often run down the old burrows of worms. The tip, however, in endeavouring to circumnutate, will continually press against the earth on all sides, and this can hardly fail to be of the highest importance to the plant; for we have seen that when little bits of card-like paper and of very thin paper were cemented on opposite sides of the tip, the whole growing part of the radicle was excited to bend away from the side bearing the card or more resisting substance, towards the side bearing the thin paper. We may therefore feel almost sure that when the tip encounters a stone or other obstacle in the ground, or even earth more compact on one side than the other, the root will bend away as much as it can from the obstacle or the more resisting earth, and will thus follow with unerring skill a line of least resistance.

The tip is more sensitive to prolonged contact with an object than to gravitation when this acts obliquely on the radicle, and sometimes even when it acts in the most favourable direction at right angles to the radicle. The tip was excited by an attached bead of shellac weighing less than 1/200th of a grain (0.33 mg.); it is therefore more sensitive than the most delicate tendril, namely, that of Passiflora gracilis, which was barely acted on by a bit of wire weighing 1/50th of a grain. But this degree of sensitiveness is as nothing compared with that of the glands of Drosera, for these are excited by particles weighing only 1/78740 of a grain. The sensitiveness of the tip cannot be accounted for by its being covered by a thinner layer of tissue than the other parts, for it is protected by the relatively thick root-cap. It is remarkable that although the radicle bends away, when one side of the tip is slightly touched with caustic, yet if the side be much cauterised the injury is too great, and the power of transmitting some influence to the adjoining parts causing them to bend, is lost. Other analogous cases are known to occur.

After a radicle has been deflected by some obstacle, geotropism directs the tip again to grow perpendicularly downwards; but geotropism is a feeble power, and here, as Sachs has shown, another interesting adaptive movement comes into play; for radicles at a distance of a few millimeters from the tip are sensitive to prolonged contact in such a manner that they bend towards the touching object, instead of from it as occurs when an object touches one side of the tip. Moreover, the curvature thus caused is abrupt; the pressed part alone bending. Even slight pressure suffices, such as a bit of card cemented to one side. therefore a radicle, as it passes over the edge of any obstacle in the ground, will through the action of geotropism press against it; and this pressure will cause the radicle to endeavour to bend abruptly over the edge. It will thus recover as quickly as possible its normal downward course.

Radicles are also sensitive to air which contains more moisture on one side than the other, and they bend towards its source. It is therefore probable that they are in like manner sensitive to dampness in the soil. It was ascertained in several cases that this sensitiveness resides in the tip, which transmits an influence causing the adjoining upper part to bend in opposition to geotropism towards the moist object. We may therefore infer that roots will be deflected from their downward course towards any source of moisture in the soil.

Again, most or all radicles are slightly sensitive to light, and according to Wiesner, generally bend a little from it. Whether this can be of any service to them is very doubtful, but with seeds germinating on the surface it will slightly aid geotropism in directing the radicles to the ground.[^[1]] We ascertained in one instance that such sensitiveness resided in the tip, and caused the adjoining parts to bend from the light. The sub-aërial roots observed by Wiesner were all apheliotropic, and this, no doubt, is of use in bringing them into contact with trunks of trees or surfaces of rock, as is their habit.

[1] Dr. Karl Richter, who has especially attended to this subject (‘K. Akad. der Wissenschaften in Wien,’ 1879, p. 149), states that apheliotropism does not aid radicles in penetrating the ground.

We thus see that with seedling plants the tip of the radicle is endowed with diverse kinds of sensitiveness; and that the tip directs the adjoining growing parts to bend to or from the exciting cause, according to the needs of the plant. The sides of the radicle are also sensitive to contact, but in a widely different manner. Gravitation, though a less powerful cause of movement than the other above specified stimuli, is ever present; so that it ultimately prevails and determines the downward growth of the root.

The primary radicle emits secondary ones which project sub-horizontally; and these were observed in one case to circumnutate. Their tips are also sensitive to contact, and they are thus excited to bend away from any touching object; so that they resemble in these respects, as far as they were observed, the primary radicles. If displaced they resume, as Sachs has shown, their original sub-horizontal position; and this apparently is due to diageotropism. The secondary radicles emit tertiary ones, but these, in the case of the bean, are not affected by gravitation; consequently they protrude in all directions. Thus the general arrangement of the three orders of roots is excellently adapted for searching the whole soil for nutriment.

Sachs has shown that if the tip of the primary radicle is cut off (and the tip will occasionally be gnawed off with seedlings in a state of nature) one of the secondary radicles grows perpendicularly downwards, in a manner which is analogous to the upward growth of a lateral shoot after the amputation of the leading shoot. We have seen with radicles of the bean that if the primary radicle is merely compressed instead of being cut off, so that an excess of sap is directed into the secondary radicles, their natural condition is disturbed and they grow downwards. Other analogous facts have been given. As anything which disturbs the constitution is apt to lead to reversion, that is, to the resumption of a former character, it appears probable that when secondary radicles grow downwards or lateral shoots upwards, they revert to the primary manner of growth proper to radicles and shoots.

With dicotyledonous seeds, after the protrusion of the radicle, the hypocotyl breaks through the seed-coats; but if the cotyledons are hypogean, it is the epicotyl which breaks forth. These organs are at first invariably arched, with the upper part bent back parallel to the lower; and they retain this form until they have risen above the ground. In some cases, however, it is the petioles of the cotyledons or of the first true leaves which break through the seed-coats as well as the ground, before any part of the stem protrudes; and then the petioles are almost invariably arched. We have met with only one exception, and that only a partial one, namely, with the petioles of the two first leaves of Acanthus candelabrum. With Delphinium nudicaule the petioles of the two cotyledons are completely confluent, and they break through the ground as an arch; afterwards the petioles of the successively formed early leaves are arched, and they are thus enabled to break through the base of the confluent petioles of the cotyledons. In the case of Megarrhiza, it is the plumule which breaks as an arch through the tube formed by the confluence of the cotyledon-petioles. With mature plants, the flower-stems and the leaves of some few species, and the rachis of several ferns, as they emerge separately from the ground, are likewise arched.

The fact of so many different organs in plants of many kinds breaking through the ground under the form of an arch, shows that this must be in some manner highly important to them. According to Haberlandt, the tender growing apex is thus saved from abrasion, and this is probably the true explanation. But as both legs of the arch grow, their power of breaking through the ground will be much increased as long as the tip remains within the seed-coats and has a point of support. In the case of monocotyledons the plumule or cotyledon is rarely arched, as far as we have seen; but this is the case with the leaf-like cotyledon of the onion; and the crown of the arch is here strengthened by a special protuberance. In the Gramineæ the summit of the straight, sheath-like cotyledon is developed into a hard sharp crest, which evidently serves for breaking through the earth. With dicotyledons the arching of the epicotyl or hypocotyl often appears as if it merely resulted from the manner in which the parts are packed within the seed; but it is doubtful whether this is the whole of the truth in any case, and it certainly was not so in several cases, in which the arching was seen to commence after the parts had wholly escaped from the seed-coats. As the arching occurred in whatever position the seeds were placed, it is no doubt due to temporarily increased growth of the nature of epinasty or hyponasty along one side of the part.

As this habit of the hypocotyl to arch itself appears to be universal, it is probably of very ancient origin. It is therefore not surprising that it should be inherited, at least to some extent, by plants having hypogean cotyledons, in which the hypocotyl is only slightly developed and never protrudes above the ground, and in which the arching is of course now quite useless. This tendency explains, as we have seen, the curvature of the hypocotyl (and the consequent movement of the radicle) which was first observed by Sachs, and which we have often had to refer to as Sachs’ curvature.

The several foregoing arched organs are continually circumnutating, or endeavouring to circumnutate, even before they break through the ground. As soon as any part of the arch protrudes from the seed-coats it is acted upon by apogeotropism, and both the legs bend upwards as quickly as the surrounding earth will permit, until the arch stands vertically. By continued growth it then forcibly breaks through the ground; but as it is continually striving to circumnutate this will aid its emergence in some slight degree, for we know that a circumnutating hypocotyl can push away damp sand on all sides. As soon as the faintest ray of light reaches a seedling, heliotropism will guide it through any crack in the soil, or through an entangled mass of overlying vegetation; for apogeotropism by itself can direct the seedling only blindly upwards. Hence probably it is that sensitiveness to light resides in the tip of the cotyledons of the Gramineæ, and in the upper part of the hypocotyls of at least some plants.

As the arch grows upwards the cotyledons are dragged out of the ground. The seed-coats are either left behind buried, or are retained for a time still enclosing the cotyledons. These are afterwards cast off merely by the swelling of the cotyledons. But with most of the Cucurbitaceæ there is a curious special contrivance for bursting the seed-coats whilst beneath the ground, namely, a peg at the base of the hypocotyl, projecting at right angles, which holds down the lower half of the seed-coats, whilst the growth of the arched part of the hypocotyl lifts up the upper half, and thus splits them in twain. A somewhat analogous structure occurs in Mimosa pudica and some other plants. Before the cotyledons are fully expanded and have diverged, the hypocotyl generally straightens itself by increased growth along the concave side, thus reversing the process which caused the arching. Ultimately not a trace of the former curvature is left, except in the case of the leaf-like cotyledons of the onion.

The cotyledons can now assume the function of leaves, and decompose carbonic acid; they also yield up to other parts of the plant the nutriment which they often contain. When they contain a large stock of nutriment they generally remain buried beneath the ground, owing to the small development of the hypocotyl; and thus they have a better chance of escaping destruction by animals. From unknown causes, nutriment is sometimes stored in the hypocotyl or in the radicle, and then one of the cotyledons or both become rudimentary, of which several instances have been given. It is probable that the extraordinary manner of germination of Megarrhiza Californica, Ipomœa leptophylla and pandurata, and of Quercus virens, is connected with the burying of the tuber-like roots, which at an early age are stocked with nutriment; for in these plants it is the petioles of the cotyledons which first protrude from the seeds, and they are then merely tipped with a minute radicle and hypocotyl. These petioles bend down geotropically like a root and penetrate the ground, so that the true root, which afterwards becomes greatly enlarged, is buried at some little depth beneath the surface. Gradations of structure are always interesting, and Asa Gray informs us that with Ipomœa Jalappa, which likewise forms huge tubers, the hypocotyl is still of considerable length, and the petioles of the cotyledons are only moderately elongated. But in addition to the advantage gained by the concealment of the nutritious matter stored within the tubers, the plumule, at least in the case of Megarrhiza, is protected from the frosts of winter by being buried.

With many dicotyledonous seedlings, as has lately been described by De Vries, the contraction of the parenchyma of the upper part of the radicle drags the hypocotyl downwards into the earth; sometimes (it is said) until even the cotyledons are buried. The hypocotyl itself of some species contracts in a like manner. It is believed that this burying process serves to protect the seedlings against the frosts of winter.

Our imaginary seedling is now mature as a seedling, for its hypocotyl is straight and its cotyledons are fully expanded. In this state the upper part of the hypocotyl and the cotyledons continue for some time to circumnutate, generally to a wide extent relatively to the size of the parts, and at a rapid rate. But seedlings profit by this power of movement only when it is modified, especially by the action of light and gravitation; for they are thus enabled to move more rapidly and to a greater extent than can most mature plants. Seedlings are subjected to a severe struggle for life, and it appears to be highly important to them that they should adapt themselves as quickly and as perfectly as possible to their conditions. Hence also it is that they are so extremely sensitive to light and gravitation. The cotyledons of some few species are sensitive to a touch; but it is probable that this is only an indirect result of the foregoing kinds of sensitiveness, for there is no reason to believe that they profit by moving when touched.

Our seedling now throws up a stem bearing leaves, and often branches, all of which whilst young are continually circumnutating. If we look, for instance, at a great acacia tree, we may feel assured that every one of the innumerable growing shoots is constantly describing small ellipses; as is each petiole, sub-petiole, and leaflet. The latter, as well as ordinary leaves, generally move up and down in nearly the same vertical plane, so that they describe very narrow ellipses. The flower-peduncles are likewise continually circumnutating. If we could look beneath the ground, and our eyes had the power of a microscope, we should see the tip of each rootlet endeavouring to sweep small ellipses or circles, as far as the pressure of the surrounding earth permitted. All this astonishing amount of movement has been going on year after year since the time when, as a seedling, the tree first emerged from the ground.

Stems are sometimes developed into long runners or stolons. These circumnutate in a conspicuous manner, and are thus aided in passing between and over surrounding obstacles. But whether the circumnutating movement has been increased for this special purpose is doubtful.

We have now to consider circumnutation in a modified form, as the source of several great classes of movement. The modification may be determined by innate causes, or by external agencies. Under the first head we see leaves which, when first unfolded, stand in a vertical position, and gradually bend downwards as they grow older. We see flower-peduncles bending down after the flower has withered, and others rising up; or again, stems with their tips at first bowed downwards, so as to be hooked, afterwards straightening themselves; and many other such cases. These changes of position, which are due to epinasty or hyponasty, occur at certain periods of the life of the plant, and are independent of any external agency. They are effected not by a continuous upward or downward movement, but by a succession of small ellipses, or by zigzag lines,—that is, by a circumnutating movement which is preponderant in some one direction.

Again, climbing plants whilst young circumnutate in the ordinary manner, but as soon as the stem has grown to a certain height, which is different for different species, it elongates rapidly, and now the amplitude of the circumnutating movement is immensely increased, evidently to favour the stem catching hold of a support. The stem also circumnutates rather more equally to all sides than in the case of non-climbing plants. This is conspicuously the case with those tendrils which consist of modified leaves, as these sweep wide circles; whilst ordinary leaves usually circumnutate nearly in the same vertical plane. Flower-peduncles when converted into tendrils have their circumnutating movement in like manner greatly increased.

We now come to our second group of circumnutating movements—those modified through external agencies. The so-called sleep or nyctitropic movements of leaves are determined by the daily alternations of light and darkness. It is not the darkness which excites them to move, but the difference in the amount of light which they receive during the day and night; for with several species, if the leaves have not been brightly illuminated during the day, they do not sleep at night. They inherit, however, some tendency to move at the proper periods, independently of any change in the amount of light. The movements are in some cases extraordinarily complex, but as a full summary has been given in the chapter devoted to this subject, we will here say but little on this head. Leaves and cotyledons assume their nocturnal position by two means, by the aid of pulvini and without such aid. In the former case the movement continues as long as the leaf or cotyledon remains in full health; whilst in the latter case it continues only whilst the part is growing. Cotyledons appear to sleep in a larger proportional number of species than do leaves. In some species, the leaves sleep and not the cotyledons; in others, the cotyledons and not the leaves; or both may sleep, and yet assume widely different positions at night.

Although the nyctitropic movements of leaves and cotyledons are wonderfully diversified, and sometimes differ much in the species of the same genus, yet the blade is always placed in such a position at night, that its upper surface is exposed as little as possible to full radiation. We cannot doubt that this is the object gained by these movements; and it has been proved that leaves exposed to a clear sky, with their blades compelled to remain horizontal, suffered much more from the cold than others which were allowed to assume their proper vertical position. Some curious facts have been given under this head, showing that horizontally extended leaves suffered more at night, when the air, which is not cooled by radiation, was prevented from freely circulating beneath their lower surfaces; and so it was, when the leaves were allowed to go to sleep on branches which had been rendered motionless. In some species the petioles rise up greatly at night, and the pinnae close together. The whole plant is thus rendered more compact, and a much smaller surface is exposed to radiation.

That the various nyctitropic movements of leaves result from modified circumnutation has, we think, been clearly shown. In the simplest cases a leaf describes a single large ellipse during the 24 h.; and the movement is so arranged that the blade stands vertically during the night, and reassumes its former position on the following morning. The course pursued differs from ordinary circumnutation only in its greater amplitude, and in its greater rapidity late in the evening and early on the following morning. Unless this movement is admitted to be one of circumnutation, such leaves do not circumnutate at all, and this would be a monstrous anomaly. In other cases, leaves and cotyledons describe several vertical ellipses during the 24 h.; and in the evening one of them is increased greatly in amplitude until the blade stands vertically either upwards or downwards. In this position it continues to circumnutate until the following morning, when it reassumes its former position. These movements, when a pulvinus is present, are often complicated by the rotation of the leaf or leaflet; and such rotation on a small scale occurs during ordinary circumnutation. The many diagrams showing the movements of sleeping and non-sleeping leaves and cotyledons should be compared, and it will be seen that they are essentially alike. Ordinary circumnutation is converted into a nyctitropic movement, firstly by an increase in its amplitude, but not to so great a degree as in the case of climbing plants, and secondly by its being rendered periodic in relation to the alternations of day and night. But there is frequently a distinct trace of periodicity in the circumnutating movements of non-sleeping leaves and cotyledons. The fact that nyctitropic movements occur in species distributed in many families throughout the whole vascular series, is intelligible, if they result from the modification of the universally present movement of circumnutation; otherwise the fact is inexplicable.

In the seventh chapter we have given the case of a Porlieria, the leaflets of which remained closed all day, as if asleep, when the plant was kept dry, apparently for the sake of checking evaporation. Something of the same kind occurs with certain Gramineæ. At the close of this same chapter, a few observations were appended on what may be called the embryology of leaves. The leaves produced by young shoots on cut-down plants of Melilotus Taurica slept like those of a Trifolium, whilst the leaves on the older branches on the same plants slept in a very different manner, proper to the genus; and from the reasons assigned we are tempted to look at this case as one of reversion to a former nyctitropic habit. So again with Desmodium gyrans, the absence of small lateral leaflets on very young plants, makes us suspect that the immediate progenitor of this species did not possess lateral leaflets, and that their appearance in an almost rudimentary condition at a somewhat more advanced age is the result of reversion to a trifoliate predecessor. However this may be, the rapid circumnutating or gyrating movements of the little lateral leaflets, seem to be due proximately to the pulvinus, or organ of movement, not having been reduced nearly so much as the blade, during the successive modifications through which the species has passed.

We now come to the highly important class of movements due to the action of a lateral light. When stems, leaves, or other organs are placed, so that one side is illuminated more brightly than the other, they bend towards the light. This heliotropic movement manifestly results from the modification of ordinary circumnutation; and every gradation between the two movements could be followed. When the light was dim, and only a very little brighter on one side than on the other, the movement consisted of a succession of ellipses, directed towards the light, each of which approached nearer to its source than the previous one. When the difference in the light on the two sides was somewhat greater, the ellipses were drawn out into a strongly-marked zigzag line, and when much greater the course became rectilinear. We have reason to believe that changes in the turgescence of the cells is the proximate cause of the movement of circumnutation; and it appears that when a plant is unequally illuminated on the two sides, the always changing turgescence is augmented along one side, and is weakened or quite arrested along the other sides. Increased turgescence is commonly followed by increased growth, so that a plant which has bent itself towards the light during the day would be fixed in this position were it not for apogeotropism acting during the night. But parts provided with pulvini bend, as Pfeffer has shown, towards the light; and here growth does not come into play any more than in the ordinary circumnutating movements of pulvini.

Heliotropism prevails widely throughout the vegetable kingdom, but whenever, from the changed habits of life of any plant, such movements become injurious or useless, the tendency is easily eliminated, as we see with climbing and insectivorous plants.

Apheliotropic movements are comparatively rare in a well-marked degree, excepting with sub-aërial roots. In the two cases investigated by us, the movement certainly consisted of modified circumnutation.

The position which leaves and cotyledons occupy during the day, namely, more or less transversely to the direction of the light, is due, according to Frank, to what we call diaheliotropism. As all leaves and cotyledons are continually circumnutating, there can hardly be a doubt that diaheliotropism results from modified circumnutation. From the fact of leaves and cotyledons frequently rising a little in the evening, it appears as if diaheliotropism had to conquer during the middle of the day a widely prevalent tendency to apogeotropism.

Lastly, the leaflets and cotyledons of some plants are known to be injured by too much light; and when the sun shines brightly on them, they move upwards or downwards, or twist laterally, so that they direct their edges towards the light, and thus they escape being injured. These paraheliotropic movements certainly consisted in one case of modified circumnutation; and so it probably is in all cases, for the leaves of all the species described circumnutate in a conspicuous manner. This movement has hitherto been observed only with leaflets provided with pulvini, in which the increased turgescence on opposite sides is not followed by growth; and we can understand why this should be so, as the movement is required only for a temporary purpose. It would manifestly be disadvantageous for the leaf to be fixed by growth in its inclined position. For it has to assume its former horizontal position, as soon as possible after the sun has ceased shining too brightly on it.

The extreme sensitiveness of certain seedlings to light, as shown in our ninth chapter, is highly remarkable. The cotyledons of Phalaris became curved towards a distant lamp, which emitted so little light, that a pencil held vertically close to the plants, did not cast any shadow which the eye could perceive on a white card. These cotyledons, therefore, were affected by a difference in the amount of light on their two sides, which the eye could not distinguish. The degree of their curvature within a given time towards a lateral light did not correspond at all strictly with the amount of light which they received; the light not being at any time in excess. They continued for nearly half an hour to bend towards a lateral light, after it had been extinguished. They bend with remarkable precision towards it, and this depends on the illumination of one whole side, or on the obscuration of the whole opposite side. The difference in the amount of light which plants at any time receive in comparison with what they have shortly before received, seems in all cases to be the chief exciting cause of those movements which are influenced by light. Thus seedlings brought out of darkness bend towards a dim lateral light, sooner than others which had previously been exposed to daylight. We have seen several analogous cases with the nyctitropic movements of leaves. A striking instance was observed in the case of the periodic movements of the cotyledons of a Cassia; in the morning a pot was placed in an obscure part of a room, and all the cotyledons rose up closed; another pot had stood in the sunlight, and the cotyledons of course remained expanded; both pots were now placed close together in the middle of the room, and the cotyledons which had been exposed to the sun, immediately began to close, while the others opened; so that the cotyledons in the two pots moved in exactly opposite directions whilst exposed to the same degree of light.

We found that if seedlings, kept in a dark place, were laterally illuminated by a small wax taper for only two or three minutes at intervals of about three-quarters of an hour, they all became bowed to the point where the taper had been held. We felt much surprised at this fact, and until we had read Wiesner’s observations, we attributed it to the after-effects of the light; but he has shown that the same degree of curvature in a plant may be induced in the course of an hour by several interrupted illuminations lasting altogether for 20 m., as by a continuous illumination of 60 m. We believe that this case, as well as our own, may be explained by the excitement from light being due not so much to its actual amount, as to the difference in amount from that previously received; and in our case there were repeated alternations from complete darkness to light. In this, and in several of the above specified respects, light seems to act on the tissues of plants, almost in the same manner as it does on the nervous system of animals. There is a much more striking analogy of the same kind, in the sensitiveness to light being localised in the tips of the cotyledons of Phalaris and Avena, and in the upper part of the hypocotyls of Brassica and Beta; and in the transmission of some influence from these upper to the lower parts, causing the latter to bend towards the light. This influence is also transmitted beneath the soil to a depth where no light enters. It follows from this localisation, that the lower parts of the cotyledons of Phalaris, etc., which normally become more bent towards a lateral light than the upper parts, may be brightly illuminated during many hours, and will not bend in the least, if all light be excluded from the tip. It is an interesting experiment to place caps over the tips of the cotyledons of Phalaris, and to allow a very little light to enter through minute orifices on one side of the caps, for the lower part of the cotyledons will then bend to this side, and not to the side which has been brightly illuminated during the whole time. In the case of the radicles of Sinapis alba, sensitiveness to light also resides in the tip, which, when laterally illuminated, causes the adjoining part of the root to bend apheliotropically.

Gravitation excites plants to bend away from the centre of the earth, or towards it, or to place themselves in a transverse position with respect to it. Although it is impossible to modify in any direct manner the attraction of gravity, yet its influence could be moderated indirectly, in the several ways described in the tenth chapter; and under such circumstances the same kind of evidence as that given in the chapter on Heliotropism, showed in the plainest manner that apogeotropic and geotropic, and probably diageotropic movements, are all modified forms of circumnutation.

Different parts of the same plant and different species are affected by gravitation in widely different degrees and manners. Some plants and organs exhibit hardly a trace of its action. Young seedlings which, as we know, circumnutate rapidly, are eminently sensitive; and we have seen the hypocotyl of Beta bending upwards through 109° in 3 h. 8 m. The after-effects of apogeotropism last for above half an hour; and horizontally-laid hypocotyls are sometimes thus carried temporarily beyond an upright position. The benefits derived from geotropism, apogeotropism, and diageotropism, are generally so manifest that they need not be specified. With the flower-peduncles of Oxalis, epinasty causes them to bend down, so that the ripening pods may be protected by the calyx from the rain. Afterwards they are carried upwards by apogeotropism in combination with hyponasty, and are thus enabled to scatter their seeds over a wider space. The capsules and flower-heads of some plants are bowed downwards through geotropism, and they then bury themselves in the earth for the protection and slow maturation of the seeds. This burying process is much facilitated by the rocking movement due to circumnutation.

In the case of the radicles of several, probably of all seedling plants, sensitiveness to gravitation is confined to the tip, which transmits an influence to the adjoining upper part, causing it to bend towards the centre of the earth. That there is transmission of this kind was proved in an interesting manner when horizontally extended radicles of the bean were exposed to the attraction of gravity for 1 or 1½ h., and their tips were then amputated. Within this time no trace of curvature was exhibited, and the radicles were now placed pointing vertically downwards; but an influence had already been transmitted from the tip to the adjoining part, for it soon became bent to one side, in the same manner as would have occurred had the radicle remained horizontal and been still acted on by geotropism. Radicles thus treated continued to grow out horizontally for two or three days, until a new tip was re-formed; and this was then acted on by geotropism, and the radicle became curved perpendicularly downwards.

It has now been shown that the following important classes of movement all arise from modified circumnutation, which is omnipresent whilst growth lasts, and after growth has ceased, whenever pulvini are present. These classes of movement consist of those due to epinasty and hyponasty,—those proper to climbing plants, commonly called revolving nutation,—the nyctitropic or sleep movements of leaves and cotyledons,—and the two immense classes of movement excited by light and gravitation. When we speak of modified circumnutation we mean that light, or the alternations of light and darkness, gravitation, slight pressure or other irritants, and certain innate or constitutional states of the plant, do not directly cause the movement; they merely lead to a temporary increase or diminution of those spontaneous changes in the turgescence of the cells which are already in progress. In what manner, light, gravitation, etc., act on the cells is not known; and we will here only remark that, if any stimulus affected the cells in such a manner as to cause some slight tendency in the affected part to bend in a beneficial manner, this tendency might easily be increased through the preservation of the more sensitive individuals. But if such bending were injurious, the tendency would be eliminated unless it was overpoweringly strong; for we know how commonly all characters in all organisms vary. Nor can we see any reason to doubt, that after the complete elimination of a tendency to bend in some one direction under a certain stimulus, the power to bend in a directly opposite direction might gradually be acquired through natural selection.[^[2]]

[2] See the remarks in Frank’s ‘Die wagerechte Richtung von Pflanzentheilen’ (1870, pp. 90, 91, etc.), on natural selection in connection with geotropism, heliotropism, etc.

Although so many movements have arisen through modified circumnutation, there are others which appear to have had a quite independent origin; but they do not form such large and important classes. When a leaf of a Mimosa is touched it suddenly assumes the same position as when asleep, but Brucke has shown that this movement results from a different state of turgescence in the cells from that which occurs during sleep; and as sleep-movements are certainly due to modified circumnutation, those from a touch can hardly be thus due. The back of a leaf of Drosera rotundifolia was cemented to the summit of a stick driven into the ground, so that it could not move in the least, and a tentacle was observed during many hours under the microscope; but it exhibited no circumnutating movement, yet after being momentarily touched with a bit of raw meat, its basal part began to curve in 23 seconds. This curving movement therefore could not have resulted from modified circumnutation. But when a small object, such as a fragment of a bristle, was placed on one side of the tip of a radicle, which we know is continually circumnutating, the induced curvature was so similar to the movement caused by geotropism, that we can hardly doubt that it is due to modified circumnutation. A flower of a Mahonia was cemented to a stick, and the stamens exhibited no signs of circumnutation under the microscope, yet when they were lightly touched they suddenly moved towards the pistil. Lastly, the curling of the extremity of a tendril when touched seems to be independent of its revolving or circumnutating movement. This is best shown by the part which is the most sensitive to contact, circumnutating much less than the lower parts, or apparently not at all.[^[3]]

[3] For the evidence on this head, see the ‘Movements and Habits of Climbing Plants,’ 1875, pp. 173, 174.

Although in these cases we have no reason to believe that the movement depends on modified circumnutation, as with the several classes of movement described in this volume, yet the difference between the two sets of cases may not be so great as it at first appears. In the one set, an irritant causes an increase or diminution in the turgescence of the cells, which are already in a state of change; whilst in the other set, the irritant first starts a similar change in their state of turgescence. Why a touch, slight pressure or any other irritant, such as electricity, heat, or the absorption of animal matter, should modify the turgescence of the affected cells in such a manner as to cause movement, we do not know. But a touch acts in this manner so often, and on such widely distinct plants, that the tendency seems to be a very general one; and if beneficial, it might be increased to any extent. In other cases, a touch produces a very different effect, as with Nitella, in which the protoplasm may be seen to recede from the walls of the cell; in Lactuca, in which a milky fluid exudes; and in the tendrils of certain Vitaceae, Cucurbitaceæ, and Bignoniaceae, in which slight pressure causes a cellular outgrowth.

Finally it is impossible not to be struck with the resemblance between the foregoing movements of plants and many of the actions performed unconsciously by the lower animals.[^[4]] With plants an astonishingly small stimulus suffices; and even with allied plants one may be highly sensitive to the slightest continued pressure, and another highly sensitive to a slight momentary touch. The habit of moving at certain periods is inherited both by plants and animals; and several other points of similitude have been specified. But the most striking resemblance is the localisation of their sensitiveness, and the transmission of an influence from the excited part to another which consequently moves. Yet plants do not of course possess nerves or a central nervous system; and we may infer that with animals such structures serve only for the more perfect transmission of impressions, and for the more complete intercommunication of the several parts.

[4] Sachs remarks to nearly the same effect: “Dass sich die lebende Pflanzensubstanz derart innerlich differenzirt, dass einzelne Theile mit specifischen Energien ausgerüstet sind, ähnlich, wie die verschiedenen Sinnesnerven des Thiere” (‘Arbeiten des Bot. Inst. in Würzburg,’ Bd. ii. 1879, p. 282).

We believe that there is no structure in plants more wonderful, as far as its functions are concerned, than the tip of the radicle. If the tip be lightly pressed or burnt or cut, it transmits an influence to the upper adjoining part, causing it to bend away from the affected side; and, what is more surprising, the tip can distinguish between a slightly harder and softer object, by which it is simultaneously pressed on opposite sides. If, however, the radicle is pressed by a similar object a little above the tip, the pressed part does not transmit any influence to the more distant parts, but bends abruptly towards the object. If the tip perceives the air to be moister on one side than on the other, it likewise transmits an influence to the upper adjoining part, which bends towards the source of moisture. When the tip is excited by light (though in the case of radicles this was ascertained in only a single instance) the adjoining part bends from the light; but when excited by gravitation the same part bends towards the centre of gravity. In almost every case we can clearly perceive the final purpose or advantage of the several movements. Two, or perhaps more, of the exciting causes often act simultaneously on the tip, and one conquers the other, no doubt in accordance with its importance for the life of the plant. The course pursued by the radicle in penetrating the ground must be determined by the tip; hence it has acquired such diverse kinds of sensitiveness. It is hardly an exaggeration to say that the tip of the radicle thus endowed, and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements.

INDEX.

A.
Abies communis, effect of killing or injuring the leading shoot, 187
— pectinata, effect of killing or injuring the leading shoot, 187
—, affected by Æcidium elatinum, 188
Abronia umbellata, its single, developed cotyledon, 78
—, rudimentary cotyledon, 95
—, rupture of the seed coats, 105
Abutilon Darwinii, sleep of leaves and not of cotyledons, 314
—, nocturnal movement of leaves, 323
Acacia Farnesiana, state of plant when awake and asleep, 381, 382
—, appearance at night, 395
—, nyctitropic movements of pinnae, 402
—, the axes of the ellipses, 404
— lophantha, character of first leaf, 415
— retinoides, circumnutation of young phyllode, 236
Acanthosicyos horrida, nocturnal movement of cotyledon 304
Acanthus candelabrum, inequality in the two first leaves, 79
—, petioles not arched, 553
— latifolius, variability in first leaves 79
— mollis, seedling, manner of breaking through the ground, 78, 79
—, circumnutation of young leaf, 249, 269
— spinosus, 79
—, movement of leaves, 249
Adenanthera pavonia, nyctitropic movements of leaflets, 374
Æcidium elatinum, effect on the lateral branches of the silver fir, 188
Æsculus hippocastanum, movements of radicle, 28, 29
—, sensitiveness of apex of radicle, 172–174
Albizzia lophantha, nyctitropic movements of leaflets, 383
—, of pinnae, 402
Allium cepa, conical protuberance on arched cotyledon, 59
—, circumnutation of basal half of arched cotyledon, 60
—, mode of breaking through ground, 87
—, straightening process, 101
— porrum, movements of flower-stems, 226
Alopecurus pratensis, joints affected by apogeotropism, 503
Aloysia citriodora, circumnutation of stem, 210
Amaranthus, sleep of leaves, 387
— caudatus, nocturnal movement of cotyledons, 307
Amorpha fruticosa, sleep of leaflets, 354
Ampelopsis tricuspidata, hyponastic movement of hooked tips, 272–275
Amphicarpoea monoica, circumnutation and nyctitropic movements of leaves,
365
—, effect of sunshine on leaflets, 445
—, geotropic movements of, 520
Anoda Wrightii, sleep of cotyledons, 302, 312
—, of leaves, 324
—, downward movement of cotyledons, 444
Apheliotropism, or negative heliotropism, 5, 419, 432
Apios graveolens, heliotropic movements of hypocotyl, 422–424
— tuberosa, vertical sinking of leaflets at night, 368
Apium graveolens, sleep of cotyledons, 305
—, petroselinum, sleep of cotyledons, 304
Apogeotropic movements effected by joints or pulvini, 502
Apogeotropism, 5, 494; retarded by heliotropism, 501; concluding remarks
on, 507
Arachis hypogoea, circumnutation of gynophore, 225
—, effects of radiation on leaves, 289, 296
—, movements of leaves, 357
— rate of movement, 404
—, circumnutation of vertically dependent young gynophores, 519
—, downward movement of the same, 519
Arching of various organs, importance of, to seedling plants, 87, 88;
emergence of hypocotyls or epicotyls in the form of an, 553
Asparagus officinalis, circumnutation of plumules, 60–62.
—, effect of lateral light, 484
Asplenium trichomanes, movement in the fruiting fronds, 257, n.
Astragalus uliginosus, movement of leaflets, 355
Avena sativa, movement of cotyledons, 65, 66.
—, sensitiveness of tip of radicle to moist air, 183
—, heliotropic movement and circumnutation of cotyledon, 421, 422
—, sensitiveness of cotyledon to a lateral light, 477
—, young sheath-like cotyledons strongly apogeotropic, 499
Avena sativa, movements of oldish cotyledons, 499, 500
Averrhoa bilimbi, leaf asleep, 330
—, angular movements when going to sleep, 331–335
—, leaflets exposed to bright sunshine, 447
Azalea Indica, circumnutation of stem, 208
B.
Bary, de, on the effect of the Æcidium on the silver fir, 188
Batalin, Prof., on the nyctitropic movements of leaves, 283; on the sleep
of leaves of Sida napoea, 322; on Polygonum aviculare, 387; on the effect
of sunshine on leaflets of Oxalis acetosella, 447
Bauhinia, nyctitropic movements, 373
—, movements of petioles of young seedlings, 401
—, appearance of young plants at night, 402
Beta vulgaris, circumnutation of hypocotyl of seedlings, 52
—, movements of cotyledons, 52, 53
—, effect of light, 124
—, nocturnal movement of cotyledons, 307
—, heliotropic movements of, 420
—, transmitted effect of light on hypocotyl, 482
—, apogeotropic movement of hypocotyl, 496
Bignonia capreolata, apheliotropic movement of tendrils, 432, 450
Bouché on Melaleuca ericaefolia, 383
Brassica napus, circumnutation of flower-stems, 226
Brassica oleracea, circumnutation of seedling, 10
—, of radicle, 11
—, geotropic movement of radicle, 11
—, movement of buried and arched hypocotyl, 13, 14, 15
—, conjoint circumnutation of hypocotyl and cotyledons, 16, 17, 18
—, of hypocotyl in darkness, 19
—, of a cotyledon with hypocotyl secured to a stick, 19, 20
—, rate of movement, 20
—, ellipses described by hypocotyls when erect, 105
—, movements of cotyledons, 115
—, — of stem, 202
—, — of leaves at night, 229, 230
—, sleep of cotyledons, 301
—, circumnutation of hypocotyl of seedling plant, 425
—, heliotropic movement and circumnutation of hypocotyls, 426
—, effect of lateral light on hypocotyls, 479–482
—, apogeotropic movement of hypocotyls, 500, 501
Brassica rapa, movements of leaves, 230
Brongniart, A., on the sleep of Strephium floribundum, 391
Bruce, Dr., on the sleep of leaves in Averrhoa, 330
Bryophyllum (vel Calanchoe) calycinum, movement of leaves, 237
C.
Camellia Japonica, circumnutation of leaf, 231, 232
Candolle, A. de, on Trapa natans, 95; on sensitiveness of cotyledons, 127
Canna Warscewiczii, circumnutation of plumules, 58, 59
—, of leaf, 252
Cannabis sativa, movements of leaves, 250
—, nocturnal movements of cotyledons, 307
Cannabis sativa, sinking of the young leaves at night, 444
Cassia, nyctitropic movement of leaves, 369
Cassia Barclayana, nocturnal movement of leaves, 372
—, slight movement of leaflets, 401
— calliantha, uninjured by exposure at night, 289, n.
—, nyctitropic movement of leaves, 371
— circumnutating movement of leaves, 372
— corymbosa, cotyledons sensitive to contact, 126
—, nyctitropic movement of leaves, 369
— floribunda, use of sleep movements, 289
—, effect of radiation on the leaves at night, 294
—, circumnutating and nyctitropic movement of a terminal leaflet, 372, 373
—, movements of young and older leaves, 400
— florida, cotyledons sensitive to contact, 126
—, sleep of cotyledons, 308
— glauca, cotyledons sensitive to contact, 126
—, sleep of cotyledons, 308
— laevigata, effect of radiation on leaves, 289, n.
— mimosoides, movement of cotyledons. 116
—, sensitiveness of, 126
—, sleep of, 308
—, nyctitropic movement of leaves, 372
—, effect of bright sunshine on cotyledons, 446
— neglecta, movements of, 117
—, effect of light, 124
—, sensitiveness of cotyledons, 126
— nodosa, non-sensitive cotyledons, 126
—, do not rise at night, 308
— pubescens, non-sensitive cotyledons, 126
Cassia pubescens, uninjured by exposure at night, 293
—, sleep of cotyledons, 308
—, nyctitropic movement of leaves, 371
—, circumnutating movement of leaves, 372
—, nyctitropic movement of petioles, 400
—, diameter of plant at night, 402
— sp. (?) movement of cotyledons, 116
— tora, circumnutation of cotyledons and hypocotyls, 34, 35, 109, 308
—, effect of light, 124, 125
—, sensitiveness to contact, 125
—, heliotropic movement and circumnutation of hypocotyl, 431
—, hypocotyl of seedling slightly heliotropic, 454
—, apogeotropic movement of old hypocotyl, 497
—, movement of hypocotyl of young seedling, 510
Caustic (nitrate of silver), effect of, on radicle of bean, 150, 156; on
the common pea, 160.
Cells, table of the measurement of, in the pulvini of Oxalis corniculata,
120; changes in, 547
Centrosema, 365
Ceratophyllum demersum, movements of stem, 211
Cereus Landbeckii, its rudimentary cotyledons, 97
— speciossimus, circumnutation of stem, 206, 207
Cerinthe major, circumnutation of hypocotyl, 49
—, of cotyledons, 49
—, ellipses described by hypocotyls when erect, 107
— effect of darkness, 124
Chatin, M., on Pinus Nordmanniana, 389
Chenopodium album, sleep of leaves but not of cotyledons, 314, 319
Chenopodium album, movement of leaves, 387
Chlorophyll injured by bright light, 446
Ciesielski, on the sensitiveness of the tip of the radicles, 4, 523
Circumnutation, meaning explained, 1; modified, 263–279; and heliotropism,
relation between, 435; of paramount importance to every plant, 547
Cissus discolor, circumnutation of leaf, 233
Citrus aurantium, circumnutation of epicotyl, 28
—, unequal cotyledons, 95
Clianthus Dampieri, nocturnal movement of leaves, 297
Cobœa scandens, circumnutation of, 270
Cohn, on the water secreted by Lathraea squamaria, 86, n.; on the movement
of leaflets of Oxalis, 447
Colutea arborea, nocturnal movement of leaflets, 355
Coniferæ, circumnutation of, 211
Coronilla rosea, leaflets asleep, 355
Corylus avellana, circumnutation of young shoot, emitted from the epicotyl,
55, 56
—, arched epicotyl, 77
Cotyledon umbilicus, circumnutation of stolons, 219, 220
Cotyledons, rudimentary, 94–98; circumnutation of, 109–112; nocturnal
movements, 111, 112; pulvini or joints of, 112–122; disturbed periodic
movements by light, 123; sensitiveness of, to contact, 125; nyctitropic
movements of, 283, 297; list of cotyledons which rise or sink at night,
300; concluding remarks on their movements, 311
Crambe maritima, circumnutation of leaves, 228, 229
Crinum Capense, shape of leaves, 253
—, circumnutation of, 254
Crotolaria (sp.?), sleep of leaves, 340
Cryptogams, circumnutation of, 257–259
Cucumis dudaim, movement of cotyledons, 43, 44
—, sleep of cotyledons, 304
Cucurbita aurantia, movement of hypocotyl, 42
—, cotyledons vertical at night, 304
—, ovifera, geotropic movement of radicle, 38, 39
—, circumnutation of arched hypocotyl, 39
—, of straight and vertical hypocotyl, 40
—, movements of cotyledons, 41, 42, 115, 124
—, position of radicle, 89
—, rupture of the seed-coats, 102
—, circumnutation of hypocotyl when erect, 107, 108
—, sensitiveness of apex of radicle, 169–171
—, cotyledons vertical at night, 304
—, not affected by apogeotropism, 509
—, tips cauterised transversely, 537
Curvature of the radicle, 193
Cycas pectinata, circumnutation of young leaf, whilst emerging from the
ground, 58
—, first leaf arched, 78
—, circumnutation of terminal leaflets, 252
Cyclamen Persicum, movement of cotyledon, 46
—, undeveloped cotyledons, 78, 96
—, circumnutation of peduncle, 225
—, —, of leaf, 246, 247
—, downward apheliotropic movement of a flower-peduncle, 433–435
Cyclamen Persicum, burying of the pods, 433
Cyperus alternifolius, circumnutation of stem, 212
—, movement of stem, 509
Cytisus fragrans, circumnutation of hypocotyl, 37
—, sleep of leaves, 344, 397
—, apogeotropic movement of stem, 494–496
+
D.
Dahlia, circumnutation of young leaves, 244–246
Dalea alopecuroides, leaflets depressed at night, 354
Darkness, effect of, on the movement of leaves, 407
Darlingtonia Californica, its leaves or pitchers apheliotropic, 450, n.
Darwin, Charles, on Maurandia semperflorens, 225; on the Swedish turnip,
230, n.; movements of climbing plants, 266, 271; the heliotropic movement
of the tendrils of Bignonia capreolata, 433; revolution of climbing plants,
451; on the curling of a tendril, 570
—, Erasmus, on the peduncles of Cyclamens, 433
—, Francis, on the radicle of Sinapis alba, 486; on Hygroscopic seeds,
489, n.
Datura stramonium, nocturnal movement of cotyledons, 298
Delpino, on cotyledons of Chaerophyllum and Corydalis, 96, n.
Delphinium nudicaule, mode of breaking through the ground, 80
—, confluent petioles of two cotyledons, 553
Desmodium gyrans, movement of leaflets, 257, n.
—, position of leaves at night, 285
—, sleep of leaves, not of cotyledons, 314
—, circumnutation and nyctitropic movement of leaves, 358–360
—, movement of lateral leaflets, 361
—, jerking of leaflets, 362
— nyctitropic movement of petioles, 400, 401
—, diameter of plant at night, 402
—, lateral movement of leaves, 404
—, zigzag movement of apex of leaf, 405
—, shape of lateral leaflet, 416
—, vespertilionis, 364, n.
Deutzia gracilis, circumnutation of stem, 205
Diageotropism, 5; or transverse-geotropism, 520
Diaheliotropism, 5; or Transversal-Heliotropismus of Frank, 419; influenced
by epinasty, 439; by weight and apogeotropism, 440
Dianthus caryophyllus, 230
—, circumnutation of young leaf, 231, 269
Dicotyledons, circumnutation widely spread among, 68
Dionoea, oscillatory movements of leaves, 261, 271
Dionoea muscipula, circumnutation of young expanding leaf, 239, 240
—, closure of the lobes and circumnutation of a full-grown leaf, 241
—, oscillations of, 242–244
Diurnal sleep, 419
Drosera Capensis, structure of first-formed leaves, 414
— rotundifolia, movement of young leaf, 237, 238
—, of the tentacles, 239
—, sensitiveness of tentacles, 261
—, shape of leaves, 414
—, leaves not heliotropic, 450
—, leaves circumnutate largely, 454
—, sensitiveness of 570
Duchartre on Trephrosia cariboea, 354; on the nyctitropic movement of the
Cassia, 369
Duval-Jouve, on the movements of Bryophyllum calycinum, 237; of the narrow
leaves of the Gramineæ, 413
Dyer, Mr. Thiselton, on the leaves of Crotolaria, 340; on Cassia
floribunda, 369, n., on the absorbent hairs on the buried flower-heads of
Trifolium subterraneum, 517
E.
Echeveria stolonifera, circumnutation of leaf, 237
Echinocactus viridescens, its rudimentary cotyledons, 97
Echinocystis lobata, movements of tendrils, 266
—, apogeotropism of tendrils, 510
Elfving, F., on the rhizomes of Sparganium ramosum, 189; on the
diageotropic movement in the rhizomes of some plants, 521
Elymus arenareus, leaves closed during the day, 413
Embryology of leaves, 414
Engelmann, Dr., on the Quercus virens, 85
Epinasty, 5, 267
Epicotyl, or plumule, 5; manner of breaking through the ground, 77; emerges
from the ground under the form of an arch, 553
Erythrina caffra, sleep of leaves, 367
— corallodendron, movement of terminal leaflet, 367
— crista-galli, effect of temperature on sleep of leaves, 318
—, circumnutation and nyctitropic movement of terminal leaflets, 367
Eucalyptus resinifera, circumnutation of leaves, 244
Euphorbia jacquineaeflora, nyctitropic movement of leaves, 388
F.
Flahault, M., on the rupture of seed-coats, 102–104, 106
Flower-stems, circumnutation of, 223–226
Fragaria Rosacea, circumnutation of stolon, 214–218
Frank, Dr. A. B., the terms Heliotropism and Geotropism, first used by him,
5, n.; radicles acted on by geotropism, 70, n.; on the stolons of Fragaria,
215; periodic and nyctitropic movements of leaves, 284; on the root-leaves
of plants kept in darkness, 443; on pulvini, 485; on natural selection in
connection with geotropism, heliotropism, etc., 570
—, on Transversal-Heliotropismus, 419
Fuchsia, circumnutation of stem, 205, 206
G.
Gazania ringens, circumnutation of stem, 208 Genera containing sleeping
plants, 320, 321
Geotropism, 5; effect of, on the primary radicle, 196; the reverse of
apogeotropism, 512: effect on the tips of radicles, 543
Geranium cinereum, 304
— Endressii, 304
— Ibericum, nocturnal movement of cotyledons, 298
— Richardsoni, 304
— rotundifolium, nocturnal movement of cotyledon, 304, 312
— subcaulescens, 304
Germinating seed, history of a, 548
Githago segetum, circumnutation of hypocotyl, 21, 108
—, burying of hypocotyl, 109
—, seedlings feebly illuminated, 124, 128
—, sleep of cotyledon, 302
—, — leaves 321
Glaucium luteum, circumnutation of young leaves, 228
Gleditschia, sleep of leaves, 368
Glycine hispida, vertical sinking of leaflets, 366
Glycyrrhiza, leaflets depressed at night, 355
Godlewski, Emil, on the turgescence of the cells, 485
Gooseberry, effect of radiation, 284
Gossypium (var. Nankin cotton), circumnutation of hypocotyl, 22
—, movement of cotyledon, 22, 23
—, sleep of leaves, 324
—, arboreum (?), sleep of cotyledons, 303
—, Braziliense, nocturnal movement of leaves, 324
—, sleep of cotyledons, 303
— herbaceum, sensitiveness of apex of radicle, 168
—, radicles cauterised transversely, 537
— maritimum, nocturnal movement of leaves, 324
Gravitation, movements excited by, 567
Gray, Asa, on Delphinium nudicaule, 80; on Megarrhiza Californica, 81; on
the movements in the fruiting fronds of Aesplenium trichomanes, 257; on the
Amphicarpoea monoica, 520; on the Ipomœa Jalappa, 557
Grease, effect of, on radicles and their tips, 182, 185
Gressner, Dr. H., on the cotyledons of Cyclamen Persicum, 46, 77; on
hypocotyl of the same, 96
Gymnosperms, 389
H.
Haberlandt, Dr., on the protuberance on the hypocotyl of Allium, 59; the
importance of the arch to seedling plants, 87; sub-aërial and subterranean
cotyledons, 110, n.; the arched hypocotyl, 554
Haematoxylon Campechianum, nocturnal movement of leaves, 368, 369
Hedera helix, circumnutation of stem, 207
Hedysarum coronarium, nocturnal movements of leaves, 356
Helianthemum prostratum, geotropic movement of flower-heads, 518
Helianthus annuus, circumnutation of hypocotyl, 45
—, arching of hypocotyl, 90
—, nocturnal movement of cotyledons, 305
Heliotropism, 5; uses of, 449; a modified form of circumnutation, 490
Helleborus niger, mode of breaking through the ground, 86
Hensen, Prof., on roots in worm-burrows, 72
Henslow, Rev. G., on the cotyledons of Phalaris Canariensis, 62
Hofmeister, on the curious movement of Spirogyra, 3, 259, n.; of the leaves
of Pistia stratiotes, 255; of cotyledons at night, 297; of petals, 414
— and Batalin on the movements of the cabbage, 229
Hooker, Sir J., on the effect of light on the pitchers of Sarracenia, 450
Hypocotyl, 5; manner of breaking through the ground, 77; emerges under the
form of an arch, 553
Hypocotyls and Epicotyls, circumnutation and other movements when arched,
98; power of straightening themselves, 100; rupture of the seed-coats,
102–106; illustration of, 106; circumnutation when erect, 107; when in
dark, 108
Hyponasty, 6, 267
I.
Iberis umbellata, movement of stem, 202.
Illumination, effect of, on the sleep of leaves, 398
Imatophyllum vel Clivia (sp.?), movement of leaves, 255
Indigofera tinctoria, leaflets depressed at night, 354
Inheritance in plants, 407, 491
Insectivorous and climbing plants not heliotropic, 450; influence of light
on, 488
Ipomœa bona nox, arching of hypocotyl, 90
—, nocturnal position of cotyledons, 306, 312
— coerulea vel Pharbitis nil, circumnutation of seedlings, 47
—, movement of cotyledons, 47–49, 109
—, nocturnal movements of cotyledons, 305
—, sleep of leaves, 386
—, sensitiveness to light, 451
—, the hypocotyledonous stems heliotropic, 453
— coccinea, position of cotyledons at night, 306, 312
— leptophylla, mode of breaking through the ground, 83, 84
—, arching of the petioles of the cotyledons, 90
—, difference in sensitiveness to gravitation in different parts, 509
—, extraordinary manner of germination, 557
Ipomœa pandurata, manner of germination, 84, 557
— purpurea (vel Pharbitis hispida), nocturnal movement of cotyledons, 305,
312
—, sleep of leaves, 386
—, sensitiveness to light, 451
—, the hypocotyledonous stems heliotropic, 453
Iris pseudo-acorus, circumnutation of leaves, 253
Irmisch, on cotyledons of Ranunculus Ficaria, 96
Ivy, its stems heliotropic, 451
K.
Kerner on the bending down of peduncles, 414
Klinostat, the, an instrument devised by Sachs to eliminate geotropism, 93
Kraus, Dr. Carl, on the underground shoots of Triticum repens, 189; on
Cannabis sativa, 250, 307, 312; on the movements of leaves, 318
L.
Lactuca scariola, sleep of cotyledons, 305
Lagenaria vulgaris, circumnutation of seedlings, 42
—, of cotyledons, 43
—, cotyledons vertical at night, 304
Lathraea squamaria, mode of breaking through the ground, 85
—, quantity of water secreted, 85, 86, n.
Lathyrus nissolia, circumnutation of stem of young seedling, 33
—, ellipses described by, 107, 108
Leaves, circumnutation of, 226–262; dicotyledons, 226–252; monocotyledons,
252–257; nyctitropism of, 280; their temperature affected by their position
at night, 294; nyctitropic or sleep movements, 315, 394; periodicity of
their movements inherited, 407; embryology of, 414; so-called diurnal
sleep, 445
Leguminosae, sleep of cotyledons, 308; sleeping species, 340
Le Maout and Decaisne, 67
Lepidium sativum, sleep of cotyledons, 302
Light, movements excited by 418, 563; influence on most vegetable tissues,
486; acts on plant as on the nervous system of animals, 487
Lilium auratum, circumnutation of stem, 212
—, apogeotropic movement of stem, 498, 499
Linnæus, ‘Somnus Plantarum’, 280; on plants sleeping, 320; on the leaves
of Sida abutilon, 324; on Œnothera mollissima, 383
Linum Berendieri, nocturnal movement of cotyledons, 298
— usitatissimum, circumnutation of stem, 203
Lolium perenne, joints affected by apogeotropism, 502
Lonicera brachypoda, hooking of the tip, 272
—, sensitiveness to light, 453
Loomis, Mr., on the movements in the fruiting fronds of Asplenium
trichomanes, 257
Lotus aristata, effect of radiation on leaves, 292
— Creticus, leaves awake and asleep, 354
— Gebelii, nocturnal movement of cotyledons, 308
—, leaflets provided with pulvini, 353
— Jacobæus, movements of cotyledons, 35, 109
—, pulvini of, 115
Lotus Jacobæus, movements at night, 116, 121, 124
—, development of pulvini, 122
—, sleep of cotyledons, 308, 313
—, nyctitropic movement of leaves, 353
— major, sleep of leaves, 353
— perigrinus, movement of leaflets, 353
Lunularia vulgaris, circumnutation of fronds, 258
Lupinus, 340
— albifrons, sleep of leaves, 344
— Hartwegii, sleep of leaves, 341
— luteus, circumnutation of cotyledons, 38, 110
—, effect of darkness, 124
Lupinus, position of leaves when asleep, 341
—, different positions of leaves at night, 343
—, varied movements of leaves and leaflets, 395
— Menziesii, sleep of leaves, 343
— mutabilis, sleep of leaves, 343
— nanus, sleep of leaves, 343
— pilosus, sleep of leaves, 340, 341
— polyphyllus, sleep of leaves, 343
— pubescens, sleep of leaves by day and night, 342
—, position of petioles at night, 343
—, movements of petioles, 401
— speciosus, circumnutation of leaves, 236
Lynch, Mr. R., on Pachira aquatica, 95, n.; sleep movements of Averrhoa,
330
M.
Maranta arundinacea, nyctitropic movement of leaves, 389–391
—, after much agitation do not sleep, 319
Marsilia quadrifoliata, effect of radiation at night, 292
—, circumnutation and nyctitropic movement of leaflets, 392–394
—, rate of movement, 404
Martins, on radiation at night, 284, n.
Masters, Dr. Maxwell, on the leading shoots of the Coniferæ, 211
Maurandia semperflorens, circumnutation of peduncle, 225
Medicago maculata, nocturnal position of leaves, 345
— marina, leaves awake and asleep, 344
Meehan, Mr., on the effect of an Æcidium on Portulaca oleracea, 189
Megarrhiza Californica, mode of breaking through the ground, 81
—, germination described by Asa Gray, 82
—, singular manner of germination, 83, 556
Melaleuca ericaefolia, sleep of leaves, 383
Melilotus, sleep of leaves, 345
— alba, sleep of leaves, 347
— coerulea, sleep of leaves, 347
— dentata, effect of radiation at night, 295
— elegans, sleep of leaves, 347
— gracilis, sleep of leaves, 347
— infesta, sleep of leaves, 347
— Italica, leaves exposed at night, 291
—, sleep of leaves, 347
— macrorrhiza, leaves exposed at night, 292
—, sleep of leaves, 347
— messanensis, sleep of leaves on full-grown and young plants, 348, 416
— officinalis, effect of exposure of leaves at night, 290, 296
—, nocturnal movement of leaves, 346, 347
—, circumnutation of leaves, 348
—, movement of petioles, 401
Melilotus parviflora, sleep of leaves, 347
— Petitpierreana, leaves exposed at night, 291, 296
—, sleep of leaves, 347
— secundiflora, sleep of leaves, 347
— suaveolens, leaves exposed at night, 291
—, sleep of leaves, 347
— sulcata, sleep of leaves, 347
— Taurica, leaves exposed at night, 291
—, sleep of leaves, 347, 415
Methods of observation, 6
Mimosa albida, cotyledons vertical at night, 116
—, not sensitive to contact, 127
—, sleep of cotyledons, 308
—, rudimentary leaflets, 364
—, nyctitropic movements of leaves, 379, 380
—, circumnutation of the main petiole of young leaf, 381
—, torsion, or rotation of leaves and leaflets, 400
—, first true leaf, 416
—, effect of bright sunshine on basal leaflets, 445
— marginata, nyctitropic movements of leaflets, 381
— pudica, movement of cotyledons, 105
—, rupture of the seed-coats, 105
—, circumnutation of cotyledons, 109
—, pulvini of, 113, 115
—, cotyledons vertical at night, 116
—, hardly sensitive to contact, 127
—, effect of exposure at night, 293
—, nocturnal movement of leaves, 297
—, sleep of cotyledons, 308
—, circumnutation and nyctitropic movement of main petiole, 374–378
—, of leaflets, 378
Mimosa albida, circumnutation and nyctitropic movement of pinnae, 402
—, number of ellipses described in given time, 406
—, effect of bright sunshine on leaflets, 446
Mirabilis jalapa and longiflora, nocturnal movements of cotyledons, 307
—, nyctitropic movement of leaves, 387
Mohl, on heliotropism in tendrils, stems, and twining plants, 451
Momentum-like movement, the accumulated effects of apogeotropism, 508
Monocotyledons, sleep of leaves, 389
Monotropa hypopitys, mode of breaking through the ground, 86
Morren, on the movements of stamens of Sparmannia and Cereus, 226
Müller, Fritz, on Cassia tora, 34; on the circumnutation of Linum
usitatissimum, 203; movements of the flower-stems of an Alisma, 226
Mutisia clematis, movement of leaves, 246
—, leaves not heliotropic, 451
N.
Natural selection in connection with geotropism, heliotropism, etc., 570
Nephrodium molle, circumnutation of very young frond, 66
—, of older frond, 257
—, slight movement of fronds, 509
Neptunia oleracea, sensitiveness to contact, 128
—, nyctitropic movement of leaflets, 374
—, of pinnae, 402
Nicotiana glauca, sleep of leaves, 385, 386
—, circumnutation of leaves, 386
Nobbe, on the rupture of the seed-coats in a seedling of Martynia, 105
Nolana prostrata, movement of seedlings in the dark, 50
—, circumnutation of seedling, 108
Nyctitropic movement of leaves, 560
Nyctitropism, or sleep of leaves, 281; in connection with radiation, 286;
object gained by it, 413
O.
Observation, methods of, 6
Œnothera mollissima, sleep of leaves, 383
Opuntia basilaris, conjoint circumnutation of hypocotyl and cotyledon, 44
—, thickening of the hypocotyl, 96
—, circumnutation of hypocotyl when erect, 107
—, burying of, 109
Orange, seedling, circumnutation of, 510
Orchis pyramidalis, complex movement of pollinia, 489
Oxalis acetosella, circumnutation of flower-stem, 224
—, effects of exposure to radiation at night, 287, 288, 296
—, circumnutation and nyctitropic movement in full-grown leaf, 326
—, circumnutation of leaflet when asleep, 327
—, rate of circumnutation of leaflets, 404
—, effect of sunshine on leaflets, 447
—, circumnutation of peduncle, 506
Oxalis acetosella, seed-capsules, only occasionally buried, 518
— articulata, nocturnal movements of cotyledons, 307
— (Biophytum) sensitiva, rapidity of movement of cotyledons during the
day, 26
—, pulvinus of, 113
—, cotyledons vertical at night, 116, 118
— bupleurifolia, circumnutation of foliaceous petiole, 328
—, nyctitropic movement of terminal leaflet, 329
— carnosa, circumnutation of flower-stem, 223
—, epinastic movements of flower-stem, 504
—, effect of exposure at night, 288, 296
—, movements of the flower-peduncles due to apogeotropism and other
forces, 503–506
— corniculata (var. cuprea), movements of cotyledons, 26
—, rising of cotyledons, 116
—, rudimentary pulvini of cotyledons, 119
—, development of pulvinus, 122
—, effect of dull light, 124
—, experiments on leaves at night, 288
— floribunda, pulvinus of cotyledons, 114
—, nocturnal movement, 118, 307, 313
— fragrans, sleep of leaves, 324
— Ortegesii, circumnutation of flower-stems, 224
—, sleep of large leaves, 327
—, diameter of plant at night, 402
—, large leaflets affected by bright sunshine, 447
— Plumierii, sleep of leaves, 327
— purpurea, exposure of leaflets at night, 293
— rosea, circumnutation of cotyledons, 23, 24
Oxalis rosea, pulvinus of, 113
—, movement of cotyledons at night, 117, 118, 307
—, effect of dull light, 124
—, non-sensitive cotyledons, 127
— sensitiva, movement of cotyledons, 109, 127, 128
—, circumnutation of flower-stem, 224
—, nocturnal movement of cotyledons, 307, 312
—, sleep of leaves, 327
— tropoeoloides, movement of cotyledons at night, 118, 120
— Valdiviana, conjoint circumnutation of cotyledons and hypocotyl, 25
—, cotyledons rising vertically at night, 114, 115, 117, 118
—, non-sensitive cotyledons, 127
—, nocturnal movement of cotyledon, 307, 312
—, sleep of leaves and not of cotyledons, 315
—, movements of leaves, 327
P.
Pachira aquatica, unequal cotyledons, 95, n.
Pancratium littorale, movement of leaves, 255
Paraheliotropism, or diurnal sleep of leaves, 445
Passiflora gracilis, circumnutation and nyctitropic movement of leaves,
383, 384
—, apogeotropic movement of tendrils, 510
—, sensitiveness of tendrils, 550
Pelargonium zonale, circumnutation of stem, 203
—, and downward movement of young leaf, 232, 233, 269
Petioles, the rising of beneficial to plant at night, 402
Petunia violacea, downward movement and circumnutation of very young leaf,
248, 249, 269.
Pfeffer, Prof., on the turgescence of the cells, 2; on pulvini of leaves,
113, 117; sleep movements of leaves, 280, 283, 284; nocturnal rising of
leaves of Malva, 324; movements of leaflets in Desmodium gyrans, 358; on
Phyllanthus Niruri, 388; influence of a pulvinus on leaves, 396; periodic
movements of sleeping leaves, 407, 408; movements of petals, 414; effect of
bright sunshine on leaflets of Robinia, 445; effect of light on parts
provided with pulvini, 363
Phalaris Canariensis, movements of old seedlings, 62
—, circumnutation of cotyledons, 63, 64, 108
—, heliotropic movement and circumnutation of cotyledon towards a dim
lateral light, 427
—, sensitiveness of cotyledon to light, 455
—, effect of exclusion of light from tips of cotyledons, 456
—, manner of bending towards light, 457
—, effects of painting with Indian ink, 467
—, transmitted effects of light, 469
—, lateral illumination of tip, 470
—, apogeotropic movement of the sheath-like cotyledons, 497
—, change from a straight upward apogeotropic course to circumnutation,
499
—, apogeotropic movement of cotyledons, 500
Phaseolus Hernandesii, nocturnal movement of leaves and leaflets, 368
—, caracalla, 93
—, nocturnal movement of leaves, 368
—, effect of bright sunshine on leaflets, 446
Phaseolus multiflorus, movement of radicles, 29
—, of young radicle, 72
—, of hypocotyl, 91, 93
—, sensitiveness of apex of radicle, 163–167
—, to moist air, 181
—, cauterisation and grease on the tips, 535
—, nocturnal movement of leaves, 368
—, nyctitropic movement of the first unifoliate leaves, 397
— Roxburghii, effect of bright sunshine on first leaves, 445
—, vulgaris, 93
—, sleep of leaves, 318
—, vertical sinking of leaflets at night, 368
Phyllanthus Niruri, sleep of leaflets, 388
— linoides, sleep of leaves, 387
Pilocereus Houlletii, rudimentary cotyledons, 97
Pimelia spectabilis, sleep of leaves, 387
Pincers, wooden, through which the radicle of a bean was allowed to grow,
75
Pinus austriaca, circumnutation of leaves, 251, 252
— Nordmanniana, nyctitropic movement of leaves, 389
— pinaster, circumnutation of hypocotyl, 56
—, movement of two opposite cotyledons, 57
—, circumnutation of young leaf, 250, 251
—, epinastic downward movement of young leaf, 270
Pistia stratiotes, movement of leaves, 255
Pisum sativum, sensitiveness of apex of radicle, 158
—, tips of radicles cauterised transversely, 534
Plants, sensitiveness to light, 449; hygroscopic movements of, 489
Plants, climbing, circumnutation of, 264; movements of, 559
—, mature, circumnutation of, 201–214
Pliny on the sleep-movements of plants, 280
Plumbago Capensis, circumnutation of stem, 208, 209
Poinciana Gilliesii, sleep of leaves, 368
Polygonum aviculare, leaves vertical at night, 387
— convolvulus, sinking of the leaves at night, 318
Pontederia (sp.?), circumnutation of leaves, 256
Porlieria hygrometrica, circumnutation and nyctitropic movements of petiole
of leaf, 335, 336
—, effect of watering, 336–338
—, leaflets closed during the day, 413
Portulaca oleracea, effect of Æcidium on, 189
Primula Sinensis, conjoint circumnutation of hypocotyl and cotyledon, 45,
46
Pringsheim on the injury to chlorophyll, 446
Prosopis, nyctitropic movements of leaflets, 374
Psoralea acaulis, nocturnal movements of leaflets, 354
Pteris aquilina, rachis of, 86
Pulvini, or joints; of cotyledons, 112–122; influence of, on the movements
of cotyledons, 313; effect on nyctitropic movements, 396
Q.
Quercus (American sp.), circumnutation of young stem, 53, 54
— robur, movement of radicles, 54, 55
—, sensitiveness of apex of radicle, 174–176
Quercus virens, manner of germination, 85, 557
R.
Radiation at night, effect of, on leaves, 284–286
Radicles, manner in which they penetrate the ground, 69–77; circumnutation
of 69; experiments with split sticks, 74; with wooden pincers, 75;
sensitiveness of apex to contact and other irritants, 129; of Vicia faba,
132–158; various experiments, 135–140; summary of results, 143–151; power
of an irritant on, compared with geotropism, 151–154; sensitiveness of tip
to moist air, 180; with greased tips, 185; effect of killing or injuring
the primary radicle, 187–191; curvature of, 193; affected by moisture, 198;
tip alone sensitive to geotropism, 540; protrusion and circumnutation in a
germinating seed, 548; tip highly sensitive, 550; the tip acts like the
brain of one of the lower animals, 573
—, secondary, sensitiveness of the tips in the bean, 154; become
vertically geotropic, 186–191
Ramey on the movements of the cotyledons of Mimosa pudica, and Clianthus
Dampieri at night, 297
Ranunculus Ficaria, mode of breaking through the ground, 86, 90
—, single cotyledon, 96
—, effect of lateral light, 484
Raphanus sativa, sensitiveness of apex of radicle, 171
—, sleep of cotyledons, 301
Rattan, Mr., on the germination of the seeds of Megarrhiza Californica, 82
Relation between circumnutation and heliotropism, 435
Reseda odorata, hypocotyl of seedling slightly heliotropic, 454
Reversion, due to mutilation, 190
Rhipsalis cassytha, rudimentary cotyledons, 97
Ricinus Borboniensis, circumnutation of arched hypocotyl, 53
Robinia, effect of bright sunshine on its leaves, 445
— pseudo-acacia, leaflets vertical at night, 355
Rodier, M., on the movements of Ceratophyllum demersum, 211
Royer, Ch., on the sleep-movements of plants, 281, n.; on the sleep of
leaves, 318; the leaves of Medicago maculata, 345; on Wistaria Sinensis,
354
Rubus idæus (hybrid) circumnutation of stem, 205
—, apogeotropic movement of stem, 498
Ruiz and Pavon, on Porlieria hygrometrica, 336
S.
SACHS on “revolving nutation,” 1; intimate connection between turgescence
and growth, 2, n.; cotyledon of the onion, 59; adaptation of root-hairs,
69; the movement of the radicle, 70, 72, 73; movement in the hypocotyls of
the bean, etc., 91; sensitiveness of radicles, 131, 145, 198;
sensitiveness of the primary radicle in the bean, 155; in the common pea,
156; effect of moist air, 180; of killing or injuring the primary radicle,
186, 187; circumnutation of flower-stems, 225; epinasty, 268; movements of
leaflets of Trifolium incarnatum, 350; action of light in modifying the
periodic movements of leaves, 418; on geotropism and heliotropism, 436,
n.; on Tropaeolum majus, 453; on the hypocotyls slightly heliotropic, and
stems strongly apheliotropic of the ivy, 453; heliotropism of radicles,
482; experiments on tips of radicles of bean, 523, 524; curvature of the
hypocotyl, 555; resemblance between plants and animals, 571
Sarracenia purpurea, circumnutation of young pitcher, 227
Saxifraga sarmentosa, circumn utation of an inclined stolon, 218
Schrankia aculeata, nyctitropic movement of the pinnae, 381, 403
— uncinata, nyctitropic movements of leaflets, 381
Securigera coronilla, nocturnal movements of leaflets, 352
Seed-capsules, burying of, 513
Seed-coats, rupture of, 102–106
Seedling plants, circumnutating movements of, 10
Selaginella, circumnutation of 258
— Kraussii (?), circumnutation of young plant, 66
Sida napoea, depression of leaves at night, 322
—, no pulvinus, 322
— retusa, vertical rising of leaves, 322
— rhombifolia, sleep of cotyledons, 308
—, sleep of leaves, 314
—, vertical rising of leaves, 322
—, no pulvinus, 322
—, circumnutation and nyctitropic movements of leaf of young plant, 322
—, nyctitropic movement of leaves, 397
Siegesbeckia orientalis, sleep of leaves, 319, 384
Sinapis alba, hypocotyl bending towards the light, 461
—, transmitted effect of light on radicles, 482, 483, 567
—, growth of radicles in darkness, 486
Sinapis nigra, sleep of cotyledons, 301
Smilax aspera, tendrils apheliotropic, 451
Smithia Pfundii, non-sensitive cotyledons, 127
—, hyponastic movement of the curved summit of the stem, 274–276
—, cotyledons not sleeping at night, 308
—, vertical movement of leaves, 356
— sensitiva, sensitiveness of cotyledons to contact, 126
—, sleep of cotyledons, 308
Sophora chrysophylla, leaflets rise at night, 368
Solanum dulcamara, circumnutating stems, 266
— lycopersicum, movement of hypocotyl, 50
—, of cotyledons, 50
—, effect of darkness, 124
—, rising of cotyledons at night, 306
—, heliotropic movements of hypocotyl, 421
—, effect of an intermittent light, 457
—, rapid heliotropism, 461
— palinacanthum, circumnutation of arched hypocotyl, 51, 100
—, of cotyledon, 51
—, ellipses described by hypocotyl when erect, 107
—, nocturnal movement of cotyledons, 306
Sparganium ramosum, rhizomes of, 189
Sphaerophysa salsola, rising of leaflets, 355
Spirogyra princeps, movements of, 259, n.
Stahl, Dr., on the effect of Æcidium on shoot, 189; on the influence of
light on swarm-spores, 488, n.
Stapelia sarpedon, circumnutation of hypocotyl, 46, 47
—, minute cotyledons, 97
Stellaria media, nocturnal movement of leaves, 297
Stems, circumnutation of, 201–214
Stolons, or Runners, circumnutation of, 214–222, 558
Strasburger, on the effect of light on spores of Haematococcus, 455, n.;
the influence of light on the swarm-spores, 488.
Strawberry, stolons of the, circumnutate, but not affected by moderate
light, 454
Strephium floribundum, circumnutation and nyctitropic movement of leaves,
391, 392
T.
Tamarindus Indica, nyctitropic movement of leaflets, 374
Transversal–heliotropismus (of Frank) or diaheliotropism, 438
Trapa natans, unequal cotyledons, 95, n.
Tecoma radicans, stems apheliotropic, 451
Tephrosia caribaea, 354
Terminology, 5
Thalia dealbata, sleep of leaves, 389
—, lateral movement of leaves, 404
Trichosanthes anguina, action of the peg on the radicle, 104
—, nocturnal movement of cotyledons, 304
Trifolium, position of terminal leaflets at night, 282
— globosum, with hairs protecting the seed-bearing flowers, 517
— glomeratum, movement of cotyledons, 309
— incarnatum, movement of cotyledons, 309
— Pannonicum, shape of first true leaf, 350, 415
Trifolium pratense, leaves exposed at night, 293
— repens, circumnutation of flower-stem, 225
—, circumnutating and epinastic movements of flower-stem, 276–279
—, nyctitropic movement of leaves, 349
—, circumnutation and nyctitropic movements of terminal leaflets, 352, 353
—, sleep movements, 349
— resupinatum, no pulvini to cotyledons, 118
—, circumnutation of stem, 204
—, effect of exposure at night, 295
—, cotyledons not rising at night, 118, 309
—, circumnutation and nyctitropic movements of terminal leaflets, 351, 352
— strictum, movements of cotyledons at night, 116, 118
—, nocturnal and diurnal movements of cotyledons, 309–311, 313
—, movement of the left-hand cotyledon, 316
— subterraneum, movement of flower-heads, 71
—, of cotyledons at night, 116, 118, 309
—, circumnutation of flower-stem, 224, 225
—, circumnutation and nyctitropic movements of leaves, 350
—, number of ellipses in 24 hours, 405
—, burying its flower-heads, 513, 514
—, downward movement of peduncle, 515
—, circumnutating movement of peduncle, 516
Trigonella Cretica, sleep of leaves, 345
Triticum repens, underground shoots of, become apogeotropic, 189
Triticum vulgare, sensitiveness of tips of radicle to moist air, 184
Tropaeolum majus (?), sensitiveness of apex of radicle to contact, 167
—, circumnutation of stem, 204
—, influence of illumination on nyctitropic movements, 338–340, 344
—, heliotropic movement and circumnutation of epicotyl of a young
seedling, 428, 429
—, of an old internode towards a lateral light, 430
—, stems of very young plants highly heliotropic, of old plants slightly
apheliotropic, 453
—, effect of lateral light, 484
— minus (?), circumnutation of buried and arched epicotyl, 27
U.
Ulex, or gorse, first-formed leaf of, 415
Uraria lagopus, vertical sinking of leaflets at night, 365
V.
Vaucher, on the burying of the flower-heads of Trifolium subterraneum, 513;
on the protection of seeds, 517
Verbena melindres (?), circumnutation of stem, 210
—, apogeotropic movement of stem, 495
Vicia faba, circumnutation of radicle, 29, 30
—, of epicotyl, 31–33
—, curvature of hypocotyl, 92
—, sensitiveness of apex of radicle, 132–134
—, of the tips of secondary radicles, 154
—, of the primary radicle above the apex, 155–158
—, various experiments, 135–143
—, summary of results, 143–151
—, power of an irritant on, compared with that of geotropism, 151–154
Vicia faba, circumnutation of leaves, 233–235
—, circumnutation of terminal leaflet, 235
—, effect of apogeotropism, 444
—, effect of amputating the tips of radicles, 523
—, regeneration of tips, 526
—, short exposure to geotropic action, 527
—, effects of amputating the tips obliquely, 528
—, of cauterising the tips, 529
—, of grease on the tips, 534
Vines, Mr., on cell growth, 3
Vries, De, on turgescence, 2; on epinasty and hyponasty, 6, 267, 268; the
protection of hypocotyls during winter, 557; stolons apheliotropic, 108;
the nyctitropic movement of leaves, 283; the position of leaves influenced
by epinasty, their own weight and apogeotropism, 440; apogeotropism in
petioles and midribs, 443; the stolons of strawberries, 454; the joints or
pulvini of the Gramineæ, 502
W.
Watering, effect of, on Porlieria hygrometrica, 336–338
Wells, ‘Essay on Dew,’ 284, n.
Wiesner, Prof., on the circumnutation of the hypocotyl, 99, 100; on the
hooked tip of climbing stems, 272; observations on the effect of bright
sunshine on chlorophyll in leaves, 446; the effects of an intermittent
light, 457; on aërial roots, 486; on special adaptations, 490
Wigandia, movement of leaves, 248
Williamson, Prof., on leaves of Drosera Capensis, 414
Wilson, Mr. A. S., on the movements of Swedish turnip leaves, 230, 298
Winkler on the protection of seedlings, 108
Wistaria Sinensis, leaflets depressed at night, 354
—, circumnutation with lateral light, 452
Z.
Zea mays, circumnutation of cotyledon, 64
Zea mays, geotropic movement of radicles, 65
—, sensitiveness of apex of radicle to contact, 177–179
—, secondary radicles, 179
—, heliotropic movements of seedling, 64, 421
—, tips of radicles cauterised, 539
Zukal, on the movements of Spirulina, 259, n.